jQuery to me was always meant as a DOM manipulation library. It didn't fall behind per se, it's just that browsers became good enough and standard enough that you no longer needed this type of tool. I think it would have been a huge mistake for jQUery to have attempted to become a Backbone, Angular or React, because it really was never meant for building web apps (of course you could still use jQuery in Backbone and Angular, and in fact those projects kind of assumed you were using it, or at least something that was API compatible).
