Question: Why is most of the information about the matter that went into forming the black hole lost?
About the article "Black hole", section "Information loss paradox".

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Because a black hole has only a few internal parameters, most of the information about the matter that went into forming the black hole is lost.
Regardless of the type of matter which goes into a black hole, it appears that only information concerning the total mass, charge, and angular momentum are conserved.
As long as black holes were thought to persist forever this information loss is not that problematic, as the information can be thought of as existing inside the black hole, inaccessible from the outside.
However, black holes slowly evaporate by emitting Hawking radiation.
This radiation does not appear to carry any additional information about the matter that formed the black hole, meaning that this information appears to be gone forever.
The question whether information is truly lost in black holes (the black hole information paradox) has divided the theoretical physics community (see Thorne–Hawking–Preskill bet).
In quantum mechanics, loss of information corresponds to the violation of vital property called unitarity, which has to do with the conservation of probability.
It has been argued that loss of unitarity would also imply violation of conservation of energy.
Over recent years evidence has been building that indeed information and unitarity are preserved in a full quantum gravitational treatment of the problem.
When particles escape, the black hole loses a small amount of its energy and therefore some of its mass (mass and energy are related by Einstein's equation "E = mc²").
The power emitted by a black hole in the form of Hawking radiation can easily be estimated for the simplest case of a nonrotating, non-charged Schwarzschild black hole of mass formula_22.
Combining the formulas for the Schwarzschild radius of the black hole, the Stefan–Boltzmann law of black-body radiation, the above formula for the temperature of the radiation, and the formula for the surface area of a sphere (the black hole's event horizon), equation derivation:Stefan–Boltzmann constant: formula_23 Schwarzschild radius: formula_24 Black hole surface gravity at the horizon: formula_25 Hawking radiation has a black-body (Planck) spectrum with a temperature T given by: formula_26 Hawking radiation temperature:Schwarzschild sphere surface area of Schwarzschild radius formula_27: formula_28 Stefan–Boltzmann power law: formula_29 A black hole is a perfect black-body: formula_30 Stefan–Boltzmann–Schwarzschild–Hawking black hole radiation power law derivation: formula_31 Stefan–Boltzmann-Schwarzschild-Hawking power law:Where formula_32 is the energy outflow, "formula_18" is the reduced Planck constant, formula_34 is the speed of light, and formula_35 is the gravitational constant.
It is worth mentioning that the above formula has not yet been derived in the framework of semiclassical gravity.
The power in the Hawking radiation from a solar mass (formula_36) black hole turns out to be a minuscule 9 × 10−29 watts.
It is indeed an extremely good approximation to call such an object 'black'.
formula_37 Under the assumption of an otherwise empty universe, so that no matter or cosmic microwave background radiation falls into the black hole, it is possible to calculate how long it would take for the black hole to dissipate: formula_38 Given that the power of the Hawking radiation is the rate of evaporation energy loss of the black hole: formula_39 Since the total energy E of the black hole is related to its mass M by Einstein's mass-energy formula: formula_40 formula_41 We can then equate this to our above expression for the power: formula_42 This differential equation is separable, and we can write: formula_43 The black hole's mass is now a function "M"("t") of time "t".
Integrating over M from formula_44 (the initial mass of the black hole) to zero (complete evaporation), and over t from zero to formula_45: formula_46 The evaporation time of a black hole is proportional to the cube of its mass: formula_47 The time that the black hole takes to dissipate is:Where formula_44 is the mass of the black hole.
The lower classical quantum limit for mass for this equation is equivalent to the Planck mass, formula_49.
Planck mass quantum black hole Hawking radiation evaporation time: formula_50 Where formula_51 is the Planck time.
For a black hole of one solar mass (formula_36 = 1.98892 × 1030 kg), we get an evaporation time of 2.098 × 1067 years—much longer than the current age of the universe at 13.798 ± 0.037 x 109 years.
formula_53 But for a black hole of 1011 kg, the evaporation time is 2.667 billion years.
This is why some astronomers are searching for signs of exploding primordial black holes.
However, since the universe contains the cosmic microwave background radiation, in order for the black hole to dissipate, it must have a temperature greater than that of the present-day black-body radiation of the universe of 2.7 K = 2.3 × 10−4 eV.
This implies that formula_22 must be less than 0.8 % of the mass of the Earth - approximately the mass of the Moon.
Cosmic microwave background radiation universe temperature: formula_55 Hawking total black hole mass: formula_56 formula_57 Where, formula_58 is the total Earth mass.
In common units, formula_59 formula_60 formula_61 So, for instance, a 1-second-lived black hole has a mass of 2.28 × 105 kg, equivalent to an energy of 2.05 × 1022 J that could be released by 5 × 106 megatons of TNT.
The initial power is 6.84 × 1021 W. Black hole evaporation has several significant consequences:  Black hole evaporation produces a more consistent view of black hole thermodynamics, by showing how black holes interact thermally with the rest of the universe.
Unlike most objects, a black hole's temperature increases as it radiates away mass.
The rate of temperature increase is exponential, with the most likely endpoint being the dissolution of the black hole in a violent burst of gamma rays.
A complete description of this dissolution requires a model of quantum gravity, however, as it occurs when the black hole approaches Planck mass and Planck radius.
The simplest models of black hole evaporation lead to the black hole information paradox.
The information content of a black hole appears to be lost when it dissipates, as under these models the Hawking radiation is random (it has no relation to the original information).
A number of solutions to this problem have been proposed, including suggestions that Hawking radiation is perturbed to contain the missing information, that the Hawking evaporation leaves some form of remnant particle containing the missing information, and that information is allowed to be lost under these conditions.  
The no-hair theorem states that, once it achieves a stable condition after formation, a black hole has only three independent physical properties: mass, charge, and angular momentum.
Any two black holes that share the same values for these properties, or parameters, are indistinguishable according to classical (i.e.
non-quantum) mechanics.
These properties are special because they are visible from outside a black hole.
For example, a charged black hole repels other like charges just like any other charged object.
Similarly, the total mass inside a sphere containing a black hole can be found by using the gravitational analog of Gauss's law, the ADM mass, far away from the black hole.
Likewise, the angular momentum can be measured from far away using frame dragging by the gravitomagnetic field.
When an object falls into a black hole, any information about the shape of the object or distribution of charge on it is evenly distributed along the horizon of the black hole, and is lost to outside observers.
The behavior of the horizon in this situation is a dissipative system that is closely analogous to that of a conductive stretchy membrane with friction and electrical resistance—the membrane paradigm.
This is different from other field theories like electromagnetism, which do not have any friction or resistivity at the microscopic level, because they are time-reversible.
Because a black hole eventually achieves a stable state with only three parameters, there is no way to avoid losing information about the initial conditions: the gravitational and electric fields of a black hole give very little information about what went in.
The information that is lost includes every quantity that cannot be measured far away from the black hole horizon, including approximately conserved quantum numbers such as the total baryon number and lepton number.
This behavior is so puzzling that it has been called the black hole information loss paradox. 
"The Black Hole War: My Battle with Stephen Hawking to Make the World Safe for Quantum Mechanics" is Susskind's second popular science book, published by Little, Brown, and Company on July 7, 2008.
The book is his most famous work and explains what he thinks would happen to the information and matter stored in a black hole when it evaporates.
The book sparked from a debate that started in 1981, when there was a meeting of physicists to try to decode some of the mysteries about how particles of particular elemental compounds function.
During this discussion Stephen Hawking stated that the information inside a black hole is lost forever as the black hole evaporates.
It took 28 years for Leonard Susskind to formulate his theory that would prove Hawking wrong.
He then published his theory in his book, The Black Hole War.
Like "The Cosmic Landscape", "The Black Hole War" is aimed at the lay reader.
He writes: "The real tools for understanding the quantum universe are abstract mathematics: infinite dimensional Hilbert spaces, projection operators, unitary matrices and a lot of other advanced principles that take a few years to learn.
But let's see how we do in just a few pages."
A nonsingular black hole model is a mathematical theory of black holes that avoids certain theoretical problems with the standard black hole model, including information loss and the unobservable nature of the black hole event horizon.