Because building attractive UIs and doing graphical things is much easier on a web stack. Emacs can run in a terminal emulator, but as an editor in a development environment, that's not a use case I've ever had. I'd much rather be able to seamlessly plot some data with vega.js or slap together a quick UI for a tool, or be able to trivially inline images, animations, videos, PDFs, etc. without having to fiddle with native libs.
This makes me wonder what an editor would look like where you can hack at the text buffer/background data/etc in something lower level, but had that information also accessible with a web based layer for doing UI stuff. Might be best of both worlds, might just be insanely messy.
That is basically the ideal editor I have in my head. It would be messy, but in the way that Emacs is also messy, which is what makes it so great :). It significantly shortens the distance from my brain to running code.
