Qt with C++ is a spectacular environment to develop UI apps in. Coupled with QtQuick it’s even better. It’s perhaps only outclassed by Flutter. As others have mentioned, there’s lots of inertia behind GTK+. There’s also past issues with licensing which made the OSS community prefer GTK+ to Qt.
I’ve no idea what’s involved in using Qt in Rust but people starting new UI apps in C and GTK+ today are likely doing a disservice to themselves and the larger OSS community that could contribute to development and maintenance.