Small correction: Brackets uses Codemirror, Visual Studio Code uses its own editor called monaco, Advanced Editor (if you mean the wordpress plugin) uses Codemirror as well, Ace is used by c9.io and RStudio for example.

I'd expect some part of their users are people who have it installed, and check from time to time if it became faster or not, but the main thing making it popular is the promise of being easily extensible.