Presumably because HP wants to control their own fate. They've seen what happened to their PC division in the race-to-the-bottom Windows world, why would they want to sign up to do that all over again with the same razor thin margins and anemic profits that every Android hardware vendor other than HTC is experiencing?

WebOS gives them a real differentiator and a chance to build an ecosystem they can grow and control, rather than just half-assing it with Android re-skin #17.