The first two answers to this quora question are from authors of each framework and give very detailed responses, I encourage you to give it a read to gain technical insight on the differences, as it will help give perspective on why Angular is more popular: http://www.quora.com/Client-side-MVC/Is-Angular-js-or-Ember-...
My personal opinion on why Angular is more popular than Ember goes back to the root difference between the two frameworks: Ember's syntax is structured in a way that favors those coming from Cocoa or Ruby, while Angular is structured in a way more familiar to those with an existing background in HTML5. I also wouldn't discredit the proximity Google already has with web developers (analytics, webmaster tools, etc).