I think it goes back to them shutting down studios when the recession happened in 2008. Since then they've been slowly building studios back up. I think we'll see much more consistent and quality releases from MS next-gen then what we've seen this gen.
The current list of studios,
343 Industries
Turn 10
Lionhead
Rare
Black Tusk
MS Victoria
Big Park
Good Science
Press Play
Soho
Lift London
Twisted Pixel
MS Los Angeles
then based on posts in this thread it appears they are building studios in Redmond and Richmond VA.
There are also partnerships with studios like Epic, Crytek, Remedy, Ruffian, and a rumored one with Respawn. If VGLeaks roadmap thing they posted a few weeks ago is true, a lot of these studios could be making core titles since it says Kinect is not the focus for gaming.