I suspect that the game won't sell itself on the Ubuntu Software Center (USC) unless it has good ratings or gets features.
That was my point, really. It's not an easy fix. It's not even really a boost. Maybe
if you sank enough time into it, you could get some sales there. You'd almost certainly be better off using that time somewhere else.
And that's the basic answer of 'why not linux'. So long as the reward from the timesink is very small, most people have better things to do with their time.
(Sorry for slow reply, I am CRAZY busy right now. Busy enough to keep even my poor linux geek too distracted to get anywhere near the Desura offerings I promised him he could make. Which also backs my point!)
For any portal in general - why on earth would you waste your time advertising THEIR site when you're already having to give away a share of profits to them? They're supposed to be bringing customers to you. That's what their profit cut is for. If they're not doing that, don't reward them!