Ignoring the unpopularity of Metro itself, Microsoft was probably in a difficult situation. The new interface doesn't support themes in any meaningful manner. I suspect a large factor are design decisions. Yet new software often lacks features of its more mature counterparts, simply because they have to ship a product in a timely manner. Seeming as Metro is flat and has lots of sharp corners, the traditional desktop had to look the same. If it didn't, people would be criticizing Windows for a lack of consistency. Even if they shipped the current theme as the default and allowed people to change the appearance of the traditional desktop, people would be criticizing the lack of consistency between applying themes.