Several reasons. Many years back a lot of distros chose to default to Gnome because of licensing doubts/FUD surrounding Qt. That decision's momentum still conti

Several reasons. Many years back a lot of distros chose to default to Gnome because of licensing doubts/FUD surrounding Qt. That decision's momentum still continues, the result being that Gnome remains default, if for no reason than to not to undergo a big shift in defaults. Secondly the ripple effects from a few major distros adopting Gnome is more pervasive than we imagine. Two of the largest, RedHat and Debian, have long used Gnome as their default desktop. And in RedHat's case, they actively contribute to and fund Gnome development. A huge number of derivative/downstream distros either go with these stock choices or have chosen to implement their own "shells" around the GTK/Gnome base. One reason for this was the historical requirement to stick to C++ for extending or modifying KDE; in Gnome's case the language support is wider when developing for it, or taking its components and building on top of it, like Cinnamon and Budgie have done. Gnome and GTK were advocated by orgs like the FSF, which has done a lot to popularise their mindshare among FOSS developers. Historically (no longer the case but momentum once established...) Gnome was also lighter on system requirements and the DE was split into small modules which could be adapted into other indie DEs. This was more difficult with KDE previously. Am sure there are more subtle reasons. But recently there's a noticeable swing towards Qt. The next version of Budgie will build upon Qt, as did the aborted Unity8. LxQt is the future LXDE. But GTK has an entrenched mindshare in FOSS that Qt just doesn't.