This is a good question. It seems like Cappucino is targeted squarely at Cocoa developers looking to do web development without retooling, just as GWT is targeted toward Java Swing developers looking to do the same thing. This can't be a large target audience. (And I imagine that the few hard-core users would probably be willing (and able) to pay something for it.)
(Haven't we all sort of agreed that the best way to do UI is largely declarative, and then we wire things together with a selector based library? Why go through the pain of having to write code just to describe static UI resources?)