IMO it's that frontend wasn't really a separate discipline at the time. Ember came out of Sproutcore, which was a framework created by some Apple MobileMe developers. They were building something that was very much an application development framework for single page apps before the term 'SPA' even existed. And I think people doing front end just weren't ready for that level of complexity yet. Most web developers were self taught and didn't have experience with UI frameworks and "real programming". At the time, doing frontend largely meant using jQuery to do ad hoc DOM manipulation and AJAX requests. Angular and then React came out and only tried to solve a subset of the problem that Ember was trying to solve, so they were easier for people to get started with. Then as time went on, it turned out that everyone did actually need solutions for everything else, but by then React had won.