Consider the game Evochron Mercenary
. It is a highly polished title which proved quite popular amongst those who played X3,Freelancer etc. The company TRIED HARD to get their game on Steam. You know what happened ? The only response they received was no.
I suspect it was the pricing. They want $30, Indie games on Steam seem to be mostly $10 or $5. Still better (from a developer perspective) than Xbox Live Indie Games which is some $3 with strong pressure to go down to $1. Also possibly user experience issues, going from this review
; having ugly indie pixel graphics is tolerable if the game is simple and fun, e.g. Cthullu Saves The World, but for this title maybe Valve thinks there aren't enough Steam users prepared to put up with simhard controls combined with Eve-style endless confusing menus.