I think Ember was a lot closer to a complete framework. I do believe that that's also why it didn't gain as much popularity. The learning curve was a lot steeper than for React or Angular. You won't run into the problems with the slimmer libraries till later and at that point you understand them and "just" learn the new thing, Redux or wherever you are including now. All along the way it feels smooth, but you end up with some more complicated, glued together whole. With a larger framework you might end up with something less complex and cleaner in the end.
I think on the backend we historically have had more complex problems sooner. You gotta connect to some database, serve multiple pages, track some context along, render views or API responses. It's much easier to anticipate the need for a framework or solutions to these problems sooner.