One theory that does make the most sense, animals that are kept in "sterile" environments from birth have weaker immune systems than snakes that have had exposure to parasites and pathogens throughout life.
This is probably the best theory, and so true. I see it time and time again, in many animals and even people (just compare my kids to their perpetually ill cousins... same allergy-prone genetics).

Did rook happen to be an amel checkered garter? I hate to say it... but I avoid keeing these guys at all because so many people have so heavily inbred them that their overall health is very poor and they often die young.
She was a regular albino checkered. They've been around for a while, but who knows how inbred they are. The less common morphs are so exciting, but the inbreeding needed to create them certainly does nothing for their health.

Now the search is on in earnest for our next garter. Something is missing without at least one.