The question is not "why not?" but "why?". Comparing a straight run of bike chain to a straight run of belt there's bugger all in it for efficiency. You've got to use hub gears if you want a geared belt-drive bike. You have to have the frame made specially for it one way or another. The belts and sprockets are wide which leads to all sorts of packaging issues. The belt is lighter than a chain but the sprockets are heavier than chainrings. You need to run lots of tension which the bearings don't like much. You have to spray it with water to stop it being all squeaky in dry/dusty conditions.
Urban commuter/utility bikes yes. Mountain bikes not yet