Is there actally a known explanation for why pokemon offspring are only the species of the mother? For example when a male pikachu breeds with a female eevee why will the babies be only eevees and none of them pikachus?

There are theories as to why this is, although there are scientists still undertaking research into that very subject. The prevailing theory is that the ovum contains all the information concerning an offspring’s species before fertilization, meaning all genetic data is donated by the mother alone, with any other information (such as inherited moves, shininess, genetic disorders, and so forth) being donated by either the father or both parents. This particular theory is favored because it’s also implicit in its support of cross-species breeding. As in, the reason why a wailord and a diglett can produce viable offspring is because the parts of the resulting hatchling’s genetic code that actually does get recombined from both parents’ gametes is technically compatible.

The less favorable theory is that species is determined by mitochondrial DNA, rather than nuclear. On the one hand, this might also explain why a pokémon’s change of species (that is to say, its evolution) is triggered by a boost in energy and radiation, as the organelle responsible for energy regulation would thus be the one responsible for encoding form and responding to the flags for form change. On the other, this is a far, far messier explanation, as mitochondria do not work like this in quite literally any other life form on Earth.