It's all dependent on vendor choice. Nothing says they have to use GTK, QT, or any other multi-platform windowing toolkit, and if they do, nothing says that the

It's all dependent on vendor choice. Nothing says they have to use GTK, QT, or any other multi-platform windowing toolkit, and if they do, nothing says that they have to build and distribute Windows or OS X versions of their software. It's just as easy to say "Windows apps written in QT are really easy to port to Linux" as the other way around. If someone writes their own code to address X11 directly, Windows wouldn't be able to run the software without an X Server, which requires the installation of a bunch of other crap through cygwin and would still probably have performance issues (that is, it'd be very much like using WINE to play Windows games, but probably much crappier). A Linux exclusive can be made, and running an unauthorized "Windows port" can easily be made to exceed the technical skillset of your average gamer.