I think most people using it are busy building stuff with it, not having time to evangelize it. I've been using it for 4 years now, for almost any type of project you could imagine. It doesn't have that shiny hype polish as the rest, but it does a perfect job for what it's supposed to, and it has the best community I've ever encountered.

It's a crazy crowded space for one thing.

Rephrase the question: "Why should mithril.js be popular?"

It's a non-starter for me because it doesn't seem to have Mobx support. I use mobx to handle my application's state at a global level.