One of Senator Bayh's stated reasons for bowing out is the increasingly partisan nature of the game.
Ok, but if Bayh is a moderate, and believes that being a moderate is for the better, what does leaving accomplish but reduce the number of moderates and throw it more over to the more purely partisan? It doesn't make sense.
I'm a Libertarian, and it's frustrating as hell being one. So, I should quit, stand aside, and assure that the Republicans and Democrats continue to destroy the country? Um, no.
I really believe one of two things, or a combination: He didn't want his wife to take arrows on the basis of her corporate board positions; and, there's some heavy dirt out there that he just didn't want to have see the light of day. Pure speculation, of course.