Bayh represents the "moderate" wing of the Democratic party that has been the primary source of the endless deal-cutting, delay, and obstruction in the Senate. "Moderate" Democrats don't hate poor people, they just love rich people, corporations, and war more. Naturally, as one of the primary causes of Senate inaction, Bayh is just plain tired of the Senate not being able to get anything done.
Bayh, son of legendary Senator Birch Bayh, has never lost an election in Indiana. And he probably wasn't going to lose this one, either. Bayh's statement blames political inaction and Terrible Partisanship, but no one actually knows why he's retiring. Maybe he literally got in bed with Big Agribusiness, and there are filthy Polaroids, or something.
Or maybe even the vague threat of a Mike Pence or someone challenging him was enough to send him into early retirement and a lucrative career as some sort of horrible hack working for some terrible company that does evil things. Like, he could join his wife in working for health insurance giant WellPoint! (Or if he wants to switch it up a bit, why not Eli Lilly?)
We'll miss you, Evan! The Senate just won't be the same without you, except that we do still have both Nelsons, Blanche Lincoln, Joe Lieberman, etc. etc..
Anyway now we are pretty sure the Republicans will retake the Senate, so, sorry if you didn't want the nation to not literally grind to a complete standstill for six years or so.