The first reason I would point, is the historical roots of patent registration. Patents appeared in a time, when designs of mechanisms of different kinds and purposes appeared during Industrial Revolution. The large majority of these mechanisms work under the laws of Classical Physics established by Sir Isaac Newton in the end of the XVIIth century. These laws are strongly deterministic and set a very straightforward view on how forces interact in a system. The construction of mechanisms based on these laws can be generally determined, as a purpose always possess a limited set of optimal combinations of forces to produce it.
Patents are generally original designs of such optimal combinations.
It should be noted that such designs are frequently successful in a long term of use. This is due to the fact that, from a limited number of forces, little can be done to create a completely different solution of the design. Consequently authors can be guranteed the right to expect a reward of their efforts for long periods of time. In the mean time, it should be noted that, with time, patents managed to cover a larger field of activities. Some scientific fields, like medicine or biology are already covered by the patent protection acts.