The default "command character" for GNU-screen is
C-a, which collides with the default key combination for
move-beginning-of-line in Emacs, and for the analogous command of many other interfaces.
It is difficult to find an alternative command character for GNU-screen that does not collide with some common Emacs binding. And for users of org-mode the problem is considerably worse.
I figure that there must be enough users of both GNU-screen and org-mode (and also of Emacs, of course) for there to be a widely accepted solution to this problem.