Because for professional development, there's really no comparison; Qt is on a different level from GTK entirely. I just did a keyword search on Monster India - Qt: 69 jobs, GTK: 3 jobs.
Yeah, just about the only reasons to use GTK+ in new projects are the plain C API and slightly better bindings. Qt bindings support deteriorated a little from Qt 4 to Qt 5 (python is there but still no java bindings as far as I can tell), whereas GTK+ isn't expanding in features so it doesn't get to have such problems.