I wasn't deeply involved in the Ember.js ecosystem as a user or maintainer, but the impression I got was that, for frontend purposes, clearer abstractions and simpler code was much more critical than "batteries included" frameworks like Rails.

Basically, if Ember.js used abstractions that were better for, say, extremely complex applications, it was dead in the water, because most applications make their library decisions when they are small and relatively straightforward. The market for javascript top-to-bottom rewrites of extremely complex apps (where something with those more complicated abstractions shine) wasn't really large enough for it to become dominant.

I also found it difficult to reason with, even though I'm an experienced Rails developer used to spooky action at a distance in the framework. Something about troubleshooting on the frontend really made it more difficult.