I haven't read the code, but I can make an guess as to why (at least one reason): Atom is using one web-renderer. React supports many version of many renderers/browser. Drop all that cruft and just go directly to the 'native' code.
This change doesn't seem to be about removing boilerplate to support multiple browsers, though. They're not implementing their own virtual dom diff their "one web-renderer", but moving away from this technology.
