My suspicion is that it started as an economic move. The Israelites were sheep herders trying to carve a niche in a dominant culture of pig and cattle herders. Telling them they couldn't eat pork and certain sacrifices had to be lambs was a way of subsidizing the sheep farmers. I came to this conclusion after reading a long chapter in the bible that talks about the breeding. Even back then they understood husbandry and economics. I doubt anyone had a clue about germ theory.