Nothing at all. Emacs is wonderful. But that says nothing at all about Ninja.
Some people just want a specialized tool for a particular job, rather than a swiss army knife that requires a lot of fiddling and presents you with a ton of stuff you can't use for your current task.
Apples, meet oranges.
The thing wrong with Emacs is that not enough people know Lisp / elisp / CL well enough to enhance Emacs and hence prefer to create new "non programmable editors" instead of enhancing Emacs : (