C bindings and APIs generally work much better in Rust because the language works a lot more like C than it does C++.
Qt depends a lot on C++ class inheritance, and even does some preprocessing of C++ files to generate code in those classes. That’s obviously not possible when using Rust. And it looks like you need a fair bit of unsafe there and there to use it at all too.
Meanwhile, GTK being a C library, its integration with Rust is much more transparent and nice.
So if you’re making a GUI Rust app, you’re just kind of better off with GTK at the moment. It’s significantly easier and nicer.