The major thing that ST has over Atom is that it's cross-platform. I develop on a Linux machine most of the time, so learning how to use Atom efficiently is completely out of the question for me until they bring out a Linux release.
Atom is a proof of concept at the moment. I don't even expect a "html5 based editor" getting prevalent.
I suspect there'll be cross-platform issues they want to solve later than sooner and thus why keeping it single platform.