>Because the phone carriers are hostile to user-freedom, and also because Plasma Active is new and immature. It's not true, though, that "nothing" runs Plasma Active, and the amount of devices it supports will increase over time. And it's not even the only current FLOSS mobile UX; there's Nemo Mobile, and there's CyanogenMod.
Care for a prediction? Plasma Active won't run on more than a handful devices with by the end of 2013. It will be used by relatively few users, mostly computer geeks. Then it'll either slowly wither away and die or will continue as a small 'niche' product.