You must work for Digia or something. If by portability you mean how well the interface looks, that is a moot point. Nobody would question that Java code was portable, and yet Java programs looked and behaved different (different dialogs, etc) than native software.
On the other hand, you fail to mention why Gtk+ is so bad in your eyes besides shiny graphics, which imho, in Linux land looks better than Qt. Why on earth would the Xfce guys care how well a Qt app looks on Windows or OSX? It is a desktop environment for X11/Wayland for christ's sake.
But in any case a post from an anonymous coward, who probably have never used either toolkit, and maybe is not even a programmer. When you have to work with this stuff, in the end you realize that it is mostly about what was best for the team at the time they started the project (availabe skillset, docs, etc) and at this point both frameworks are the best the open source world has to offer. If you don't enjoy diversity you can go back to Win32, lol.