Atom is in competition with Brackets (Ace), Visual Studio Code (CodeMirror), Advanced Editor (Ace), to cite only the free software. All the others are quicker. I wonder how they get all these users.
Atom, from github has no "we also make" worries the developers might wedge in there.
VS Code was last to the party.
I still use brackets but I installed Atom last week to have a go with. I was pleased with what comes out of the box. (context): I used to use sublime since (roughly) just before 3.0 builds were announced then used brackets from around 1.0.
sublime and brackets used to really crawl until I excluded things like .git and node_modules form their search tools. Perhaps Atom is similar for people?
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.