Nothing...it's that the app sacrifices new user accessibility for a more powerful editing paradigm. It requires a dedication to productivity from the user that pays off in the long run.
Then again, I have a sneaking suspicion that it has to do with people liking the exclusivity factor.
They both come from a day when all UI's were text based and so it's not surprising that they offer more powerful editing models than newer alternatives, considering they've both had 30 odd years of refinement.
It's also understandable that people find them more difficult to use. The major advantage that a GUI has over a text interface is that it's more discoverable and given that most people these days have grown up around GUI's, we thus find text based UI's intimidating.
Either way, as developers, most of our day to day tasks revolve around editing text, so I'd say that using a tool that's aimed at giving us maximum textual love is a good thing.
And don't get me started on asking why, after all this time, emacs still defaults to non-CUA when it has a perfectly good mode that emulates it? That's just asking to keep new users away.