Sort of similar to what Zelphyr was saying. We liked the idea of sort-of building our own framework out of component parts, rather than something like Angular, mostly for the flexibility.

For example, we use Sammy for client-side routing. However, if we decided to ditch it and use something else in the future, it wouldn't be a huge effort to do that, theoretically. With something like Angular, though, it's their way or the highway, usually - you couldn't use a different routing system if you even wanted to.

This lets us pick and choose with the style we want instead of having to 'fight with' a tool trying to work around parts we don't like.

I don't know, I've also felt like recently we've been re-inventing the wheel to get all this stuff to work together anyway, so I'm not 100% sure of the value of our choices, but it's a conscious choice we made early on in any case.