I think the amount of work that goes into them, regularly I hear 12-18 months, and that they aren't usually engineered towards intuitive processes or interfaces. I imagine features rather than experience is a driver.
The biggest driver is that I want it to work equally well on mobile, including phone sized screens.
The other is the file size and load speed. Right now the build is pushing almost 3mb, and takes around 10-20 seconds to load. That's kind of ok, since the workspace is intended to be a very long running app that folks will use all day long, but it still really sucks. You can view a demo of the ExtJS workspace at: http://stockor.com/demo
Plus it's just kind of ugly in my opinion. It looks like a desktop app that's attempting to be delivered in a web browser, which fair enough it is. I'd like it to be something more stylish though.
As for why I'm picking Backbone vs Ember or Angular, that's a bit more complicated. The main reason is that I'm very familiar with Backbone and have used it for years (I'm a contributor to DocumentCloud).
When I made the decision to rewrite for mobile, I did do a pretty in-depth framework comparison and Backbone still comes out ahead in my opinion. It's light enough that it's super easy to understand and figure out why things aren't working the way you like, while giving you just enough structure to keep the app on the correct path. I am going to use a view/model binding lib though, but haven't settled on which one as of yet. Probably backbone.stickit.
Email me at the address in my profile if you'd like to chat more about it, glad to answer any questions you might have.
