IMO, increased toleration of humans goes along with captive breeding. As the more tolerant of humans, the more likely to breed in captivity. On the other hand, captive raised snakes (and dogs and cats) are more tolerant of humans than wild caught adults. So separating the effects of heredity and environment can be difficult.

Captive breeding also tends to dumb down a species. Look at the intelligence of domestic turkeys vs. wild turkeys, for example.