I'm guessing that most of the guts of the software will work anywhere but polishing the frontend is a platform unique job. Github probably has a majority OSX userbase so this made sense for the initial release.
Although, it's based off on Chromium. So, there's no need for much "polishing" since most of it is just HTML, CSS & JS.
There are a number of things: packages that use native modules, menu concerns, app packaging, etc.