From the economical point of view, KSP has to launch on Steam. Normally, if you write a piece of software, you try to reach an audience as large as possible to get as much money as possible. And the argument, that you need to launch Steam to play it, is absurd. It was already mentioned, that M&B doesn't need this. You can add nearly every game which was released in a Humble Indie Bundle, since 80-90% of these games can be activated on Steam but have also a DRM-free installer (criterion for HIB).