[3pt14159]

It's more the type of app that you build with Ember. It ends up just being so much more complex and that requires multiple requests. You can't locally store everything, and you often can't do everything on the first request, so if you're on a connection where packet loss is an issue instead of 3 or 4 round trips you end up having 16 or 20. With a shitty device the commutative lag between janky animations (which ember makes easy with libraries like Ember-Concurrency) it just destroys the experience.

Whereas the other frameworks don't make things as app-y as easily so its more like a local template language than an app builder. So people don't go as hog wild.

That plus a larger initial JS file than something slimmed down like React. It kinda adds up even if in theory it shouldn't.