Because, in my opinion, Qt (and particularly QML/QtQuick) is a much, much better choice for building modern applications for several reasons:
1) It's the framework of choice for just about every major Linux on mobile project recently, including Ubuntu Touch, Sailfish OS, and Plasma Mobile. It's also very cross-platform and works on iOS/Android as well.
2) It's very easy to build highly polished mobile and convergent applications.
3) Qt is written is C++, while Gtk is written in C. Having worked with both C and C++, C++ is worlds better and more fun to work with.
4) It's much easier to extend QtQuick for Material design than trying to do the same with Gtk.