Qt has widespread acceptance as it is dual licensed. For proprietary apps you have to pay, but if you want to develop OS apps its free. It is the best of both worlds.
Add that to the fact that you can practically make an app that compiles and works (mostly) an all 3 major oses...Many kde apps compiled and worked out of the box on osx and windows and ultimately is something that could bring many more developers to linux.
Urgh didn't want to come across as a salesman, but ultimately it works out better than gtk (which suffers somewhat from lack of resources).