I think this is more inertia than anything else. It is pretty obvious that Stallman isn't the easiest person to talk to, but he has consistently put huge amount of efforts into GNU for lots of years, and, whatever he did as the maintainer, seems to have worked out pretty much in the end. If there was anyone as active in the Emacs community today as Stallman was in his prime, they could easily push through changes despite Stallman's disagreement, via a fork or otherwise, but there isn't such a person, and that is that.

Consider that it has been almost 30 years since GNU Emacs started, for most of this time RMS has been the maintainer, and GNU Emacs continues to this day to be the most advanced, popular and active Emacs there is, while the various forks in existence like XEmacs, SXEmacs lost steam pretty quickly. So it certainly hasn't been all bad on his side.