I use a 1HB now, so my analysis might not be the best, but...
From what I know, most modern 2HB are pretty much your non-dominant arm's forehand + your dominant hand just sitting there for control. Further up the handle you go, the harder it is to hit that non-dominant forehand.
I'd also say it's a control thing. Easier to control a racket when your hands are closer together than further a part.