It is used for even more than that. KDE, the Linux desktop environment with its huge set of applications has used Qt since its early releases in the 90s. There is also a large set of commercial desktop applications for Windows and macOS that use Qt. That Qt is used for embedded software UIs is a comparably recent development in its long history.

I think the reason for this is that Qt has extremely high quality widgets and graphical elements. They allow you to do things out of the box that would be crazy hacks with most of the competitors. And the options for styling the UIs are very good, too, which matters a lot for these embedded use cases.

Is Qt hard to get into? Yes and no. In C++, you are required to use some mechanisms like signals/slots, which are used for routing events. These features look like non-standard language extensions and rely on a special source code preprocessor to generate the glue code that is implied. The other concepts are easier to pick up. And the toolkit is very consistent in the structure of its interfaces, so once you pick up the patterns, you can get very productive.