The real answer, which is somewhat saddening given the immense amount of effort put into Stratolaunch, is that air-launch, when all is accounted for, has negligible benefits from a performance perspective when you are deploying expendable stages.

AIAA paper 2004-872 "Air Launching Earth-to-Orbit Vehicles: Delta V gains from Launch Conditions and Vehicle Aerodynamics" gives a decent and readable treatment of this scenario.

The advantages air-launch inarguably has are letting you pick the location and azimuth at which the high-performance launch vehicle stage begins its run as opposed to being constrained by your launch site and populations under the launch vehicle trajectory. If those are your most important metrics, air-launch may "win". If you are releasing a reusable stage from the air-launch platform, that could also be a unique value proposition as you could do a downrange landing of the stage (reserve no propellants for boost-back) to any desired site. SpaceX accomplishes this with a landing barge for their system.

By virtually any other measures, air launch from a subsonic platform does not make engineering sense over a ground-launched system. DARPA's ALASA program explored air-launch from an F-15, and was struggling to get ~100 lbs. to orbit. Even if you use a "big" supersonic platform like a B-1, by the time you take into account the volume associated with a launch vehicle, you quickly realize that at absolute best, you are talking a low supersonic staging Mach number. It would require a new high-thrust aircraft platform if you wanted a supersonic air-drop. And regardless of staging speed, if the flight path angle ("gamma") at release is too low, the deltaV advantage significantly diminishes (I recall the cited paper above shows this).

Consider the following issues:

* The Strato carrier aircraft is (unless even more megadollars are spent) is one-of-a-kind. If anything happens to it, no paying customer will be able to launch.

* Any stage stack that is launched from the carrier aircraft has to be rated for manned safety. This is its own expensive undertaking that autonomous ground-launched systems do not have to contend with. This is in addition to any public safety issues associated with population overflight (to be fair, air-launch may be able to diminish those concerns, but compliance with the former is much more onerous than the latter).

* The carrier aircraft presents very significant structural and volumetric constraints to the launch vehicle. The rumored lift capability is in the neighborhood of 500 klbs. For something like a hydrogen-fueled launch stack, you can do some rough sizing and realize that geometry will constrain you faster than mass. Don't forget to include flexible body effects, as that will be very significant with this airframe.

There is definitely an engineering accomplishment in the flight of this aircraft. However, unless Strato does something that they can uniquely do, or they lose money hand-over-fist on each launch (Pegasus is very expensive already, and I think SpaceNews reported that Strato shut down their own internal launch vehicle development efforts), they cannot be commercially-competitive. That launch point and azimuth control has to be critical, or air-drop just doesn't make sense.

As much as Burt Rutan has to be admired for his ability to think creatively, Stratolaunch was an insufficiently vetted architecture sold to an enthusiast billionaire who was probably delighted with the result of their SpaceShip One collaboration. And yet you see that in SS1, Scaled basically designed a one-off experimental vehicle architecture that fared very poorly when it was scaled into what was intended to be a sustainably-operated commercial system. Strato is another example of this kind of mindset.