Emacs users (myself included) would feel less like cramming every aspect of our work into Emacs if more tools embodied the freedom and hackability that comes with using Emacs. It's not that Emacs is better, it's that other tools are more restrictive and not self-documenting.

It's also helpful that you get a certain degree of UI consistency from things like completing-read being used everywhere. And even when some package departs from convention, everything is still a buffer, so anything in it is easy to reuse.