Because firstly, it's OpenGL. It's damn easy to port it to Linux. Second, it's not that type of a game that you're used to see. Their market is everybody that uses a browser, thus having it run on every computer is a lot more important. Not that I think they would port it to Linux if porting it to OS X wouldn't be half of the job ;)

Try registering, installing and playing it. It's build to be as simple as it can. That's why, I think, marketing it as a browser game is a great idea, as it just sounds like you can try it in a few minutes.

Is that they'll be testing the versions soon. I don't get why the header is "Mac and Linux are "Top Priority", as they said it a month or so before.