Ember shines best when building very UI rich, complex front end apps. Not apps that need to work on shitty connections or devices. Apps you would use from your office in NY or Toronto.
But the sad fact is that there just aren't that many frontend apps out there, relatively speaking. Ember is hard to learn because you need to learn a lot to get things done, and most people building something simple are lazy. By the time their project is a huge mess they're already locked in. Take HN for instance, the tech powering it is dead simple. It doesn't need to be giant frontend app.
Ember is a really solid framework that I love that is never going away, but it isn't for everyone and that is ok. Ember devs make a higher salary because they generally need to know more and the apps are more maintainable and generally run by pro dev shops, but it's not for everyone.
One thing I think that could really help Ember is a version of Rails or some other backend that required less mucking around to get everything working. It's such a chore to fully follow JSON API v1.1. But relation links and cursor based pagination are great with tools like Ember Data, so it's worth the effort.