Questions about crossbreeding/hybrids
 

Why is it that ball pythons are very often mixed and matched together, but milk snakes are rarely ever bred with other types of milk snake? Like a pueblan x Nelson's cross is just never done?
And then milk x King hybrids are even more rarely done, and frowned upon... Why?

Like, corn and rat snakes are very often hybridized.. ball pythons have countless morphs that are mixed together..

Morphs are different than species/subspecies.

Polymorphic combinations (as in, e.g. ball pythons) are not hybrids.

They are frowned upon largely because when they are resold, or bred, and the offspring sold and resold, people end up with snakes that aren't what they think they are. Their actual lineage is forgotten and/or misrepresented.
Most serious snake breeders prefer to know what they have, and don't like the idea of their lines potentially being messed up by unknown hybrids.
Many people also see little point to hybrids, as you are losing the unique features of both parents.
Conversely, ball pythons are all the same species, as far as we currently know. Although most ball python breeders prefer not to have unknown genetics either.