I just watched this [1] video from Dirk Hohndel describing the migration motives and process from Gtk+ to Qt for subsurface (started by Linus).

TLDR:

* Qt has a community of people developing applications, has good documentation (precise, good coverage) and people who care

* Gtk+ has a bunch of Gnome developers, if you want to develop an application, you're out of luck

[1]: https://www.youtube.com/watch?v=ON0A1dsQOV0