My impression is that while Qt has quite widespread adoption, KDE is still pretty niche. The majority of that I'd guess is that Qt makes a point to be cross-platform. Gtk since 3.0 is pretty well Linux-only and content to stay that way.
Gnome is still the default in most of the big distributions -- but arguably its most popular incarnation, Ubuntu's, deviates significantly from upstream's vision for the default Gnome desktop.
Some of these things are pretty hilarious to watch from the outside. For design/UX considerations, Gnome 3 hasn't supported desktop icons in a while now -- so a bunch of distributions ship it with one of the (several) desktop icon extensions, none of which are very functional, along with a bunch of other extensions, like Dash-to-Dock. So every time an unsuspecting user gets a work-issued Ubuntu laptop, you get to watch them trying to figure out how to drag a folder to the desktop, or pin a folder to the dock or whatever.
