I have always wondered the same thing; Microsoft is one of the very few companies that is capable enough to resist Valve's dominance. I think all it boiled down to was that in this case Microsoft, by some miracle, actually managed to catch an accurate picture of the wants and needs of their consumers and realised backing down on their Xbox store can easily drive them away. They saw how the general trend of game launchers has mostly only served in making lives more difficult for gamers and decided they'd rather keep the status quo as it is comfortable to them for the time being.