Angular is seen as "The Google Solution" (disregarding that Google doesn't actually use it as much as people think). React is seen as "The Facebook Solution". Ember is more decentralized, and lacking a strong primary champion/sponsor. Something that developers SAY we like in open source projects, but which frankly doesn't really match our actions so much.
I chuckled a bit at the parent comment, but I won't say that logos don't matter to front-end developers (half of the HN discussion threads about a link seem to be about its font and CSS choices rather than its substantive content). But that's only a slice of what I think is the overall reason. Ember just doesn't have strong corporate brand muscle behind it.
I think the react 'view as a function of state' makes a ton of things easier to reason about, and will probably last for a while.
But it is.
A lot of how "clean" or "modern" people think a given library is, is driven almost entirely by it's marketing homepage and only loosely tied to the actual experience of using it.
When we changed, I wrote about it: https://about.gitlab.com/2015/05/18/a-new-gitlab-logo/
This was their downfall.
Angular, sexy geometric design. React, sexy nuclear design. Vue, Sexy V geometric design. Ember, a big goofy rodent looking at you lol. "But abledon, what about GOlang? they have a rodent too?" Yes but golang resides in backend land where designers and frontend visual brains are lacking, only performance matters, doesnt matter if your framework's logo is a steaming pile of poo, if its fast and easy to write concurrent code in, it will win the backend holy wars.