Because it's a closed-source Android variant that only works on specific devices.

Edit: Actually, that's beside the point. I think the reason they made Honeycomb is because they thought a lot of people would buy Android tablets, because a lot of people were buying iPads. But they were wrong about that.

I'm just saying, if they did it this way one time, there's no guarantee they won't do it again. How "open" it is, is really subject to Google's whim.