Atom is a text editor, and text editors have an insanely high bar to clear in terms of performance and responsiveness. Users will abandon a text editor if the cursor takes a bit too long to move. On top of that, Atom has been criticized about its slowness since the very first announcement. They don't have any margin for error there (and to be honest, I think their technical choice of going for Javascript will be their ultimate downfall, but that's a discussion for another day).
Also, as it was pointed out, Atom didn't really embrace much of React to start with (which is to their credit: always be very conservative when you're adopting a bleeding edge, unproven technology).
I think React has potential. It's at about the same stage of maturity that Angular was five years ago, and if it's as successful, we can expect it to enjoy five years of being the new darling in the Javascript world, until the Next Big Framework comes around.
I'm really enjoying how fast Javascript frameworks and practices are churning, it makes me feel like I'm witnessing the birth of a brand new software field with my very eyes.