No one's gonna answer the question from this thread?
Well, I think Microsoft advertises their games better, and regardless of their quality (which they sure have), they seem to appeal more to the younger audience (I mean 13-19yr), at least in the US. Now try to explain to a 15yr old teen why he should play Uncharted instead of Gears...(Gears definitely sounds more "epic"). Try to tell someone how LBP is better than Halo (no chance). etc.