Windows is just too resource hungry
They can try and get it into as many devices as possible, but Windows requires more resources than alternatives.
Also, the alternatives are often real time capable. You don't want an OS pausing for a brief second when it is controlling something that needs to react quickly.
As for the surface idea, they need to redesign the way the GUI works to make it work with the controls. Maybe the screen could be sub-divided and you can align all running apps into squares on the screeen, switch between them by pressing a button and touching the app you want to make it full screen. Apple has something like this in Mac OSX.