That should be obvious: cross-platform GUI toolkits are so hard that there isn't any that you like. It takes a good design or lots of people working on it full-time. So either you pay for Qt or deal with other problems. That's one reason that web programming has become so popular.

There seems to be another factor if you add OS-X to the required platforms: the way of GUI programming for Macs is conceptually different from Windows or Linux.