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.

As I understand it, QT is good for embedded applications because it uses a very direct, weakly/efficiently abstracted API. You can do a lot with minimal computing resources. Putting something like chrome or firefox's rendering engine in embedded devices means carrying across lots of costly bloat or doing absurd work to slim them down. The only thing that might make QT hard to get into is the lack of enthusiasm for it, the sparseness of expertise, stagnant job market, etc. Things that are "not hip" in development usually have similar obstacles.