Qt has issues with lifetimes of objects. It's easy to shoot yourself in the foot by accessing objects that have already been destroyed. The borrow checker can solve that, but still shows that C++ was not really the best choice as a language for building UIs.
I also think that the success of the combination of Qt/C++ is mostly a result of the evolution of programming environments. C++ was massively popular, so that could explain it.
I also think that the success of the combination of Qt/C++ is mostly a result of the evolution of programming environments. C++ was massively popular, so that could explain it.