This is about how long people stay at the same job, not the framework stability. In my experience people hop from job to job often enough that you'd be regularly training people on a language unless they already knew it when hired. So a language that people already know or has a low learning curve is usually preferred for many companies.
Errr, Ember is the most stable of currently used JS frameworks. And generally this whole "change JS framework every year" meme is crazily overblown.