I always preferred GTK and its ecosystem. But GTK is more and more positioning itself as just a toolkit for GNOME. And GNOME is more and more going not only going its own way but also not concerning itself with compatibility beyond what I will call the Red Hat Linux platform.
So I can certainly see why anybody outside of GNOME would consider migrating off GTK. I mean, this is basically how we got COSMIC as well, based off the Iced toolkit.