I think there’s a pretty big thing here that people are kinda missing and it’s the ease of app creation. Recently gnome has done a lot for app developers to make the experience really good. Workbench, gnome builder, biblioteca all combined with the ease of libadwaita and the gnome circle make app development significantly easier for gtk than qt. There’s a big community now with a lot of inertia. I think workbench has around 100 tutorials now or so. Super low barrier to entry. Also libadwaita is pretty.
I already use gnome now, but even before I had settled on a DE, I took a look at both ecosystems and was heavily leaning towards gtk because of all these factors and against qt because of its reliance on c++. Until all of those factors are replicated well for qt (and by extension kde), I don’t think they’ll see the same level of development.