Similarly, I find a lot of left wing homies play Imperial Guard (it me, I like all the little guys) but the Guard players with the worst politics invariably play Krieg

I think the presence of space marines draws away the people looking for power fantasies of various kinds (from your black templars to your salamanders) and leaves the people who's fantasy is either ordering people to their deaths (Krieg) or standing tall with your comrades against overwhelming odds (most other regiments). Of course, the less said about the fantasies of Praetorian players, the better.