Qt has much more compact packaging - few dlls, maybe plugins, and even static linking maybe possible for GPL or other free software.
While I love gtk for the fact that it's C, it's not that easy to use from ffi-exposed systems like luajit, since it relies too much on preprocessor macro magic that has to be replicated there...
We wanted to please the trendy Mac users, hence switched to Qt, which providers a more authentic interface on Mac OS X.