What evidence is there of this? Who are the "users" of GTK that you're referring to? Moving an existing GUI app between toolkits is typically almost equivalent to a complete rewrite, so applications (which are the real "users" of a toolkit) generally do not move. Developers may start new projects using Qt having previously used GTK, but who counts this? How would we judge if it is significant?
Qt has as many issues as GTK once you start using it for complex, deep applications. Different issues, to be sure, but no GUI toolkit gives you a free ride.
Qt is also developed using a different licensing/income generation model than GTK, which changes quite a lot.
Qt mostly has distinct advantages over GTK, and to be honest if I was starting cross-platform development now (22 years after I actually did), I'd probably pick Qt for all the obvious reasons. But it's fairly pointless to ask "how can GTK be more like Qt?" when there's more or less no chance or pathway for that to happen. As it is, I don't do mobile so GTK's issues there don't affect me. I also have 75k lines of code that would have to be almost completely rewritten to use Qt, with no noticeable benefit for our users and only marginal benefits for our developers.
Speaking of "why can't?", why can't I write a C application using Qt ? :))
