Great question...
In short... it's just not ready yet.
Our current plan (subject to change) is to start selling "early, early access" or "open alpha" here on this site first (which will be a Steam code, so as if you had bought it via Steam), continue improving and iterating for a few more months, and then launch on Steam as "Early Access".
Currently we're in the middle or a major refactoring of the collision system and we've decided to not wait and implement a material system now... so soon you'll be able to construct planets out of various ratios of iron, rock, water, and hydrogen/helium.
We do not yet have a date for "open alpha" but it will be before the end of 2014.
Topic: Why Universe Sandbox 2 is not on Steam with early access? (Read 4568 times)
