What's the reason for all their other rules? People fixate on pork because that's the one rule which seems to have a rational basis, but the Jewish religion (and others) have tons of rules which are much wackier. What's the basis for their rule banning the mixing of wool with linen, but permitting either or by themselves?

Probably it was some ancient schizo that got his harebrained nonsense written down, or it was just a power flex over adherents as cults do, or some mix of the two. Assuming that these religious rules must have an underlying rational basis is foolish. Given the context of the pork rule, being surrounded by hundreds of completely wacky rules, it seems most likely to me that the pork rule was completely arbitrary and turned out to be "correct" by accident.

It's currently unknown where the prohibition came from. It seems to have appeared during the finalization of the Torah sometime between the second and fifth centuries BCE. Pigs were widely grown and eaten in that period.