It's tech hipsterism. Plus, it's a quick-and-easy way to create a hierarchy in an industry where somebody who's been working for a decade has the same job title, dresses the same, and looks basically the same as somebody who's in their second year. "Oh, you still use jQuery? Yeah, I pretty much just write pure JS." It's ridiculous tech dick measuring (which is to say it's par for the course when you get three nerds together in a room).

It's the exact same impulse that says you're not a real dev if you use language X or framework Y.