I don't know whether this is an insult or a compliment to ZK. ZK is quite popular, but I agree that it could(and should) be more popular - perhaps at the level of Struts/JSF/Wicket etc.
It is the easiest framework to learn, it offers the best default UI controls of any framework (across Java/Ruby/PHP), the framework itself is quite lightweight, it has excellent free documentation, and the forums are very active with good support from the ZK developers.

The only negative that I have heard about ZK is that it is slow (I don't think so) and can potentially use a lot of memory on the server if there are too many concurrent users of the application. I am not sure how true this is, and how it compares to other component frameworks (such as Tapestry/JSF) in this regard. Some comparison metrics (with JSF/Wicket/Tapestry) from the ZK team would help in this regard.