what is it that you think they are not pushing? games in general? my experience has been fairly bleak with the games I've published, but the games i published were buggy / underperforming ports from GameSalad. I also really didn't (and honestly still don't) have any real insight into how to market Win8 or WP8 games.
I think MS could do more, but i think they are doing what is reasonable. It's not their job to market games. it's the developer. they have their top games and give popular apps exposure. They could step up the store to be more like Apple and various Android markets allowing showcases, app of the day, etc.
As for why these tablets don't do better.. I do think that while win8 has made an effort to bridge the gap between a pc and tablet... the UX is still not there. My wife has my Samsung Slate which is an i5 CPU and runs Win8.1 She uses it for facebook, web and email... and simply put, not everything is really well designed for touch.. sure its possible, but its awkward..
I'm not saying this is an easy problem to solve, but until they get it right, people are not going to be giving up their iPads and Android tablets that understand that touch UX must be frustration-free. Once, MS does this, I think they will be on to something. The fact that no one else has tried only reinforces that this is not an easy undertaking.