Because it's the most popular & flexible cross platform user space compositor:
- unlike QT it's free for commercial use.
- I don't know any other user land GUI toolkit/compositor that isn't a game engine(unity/unreal/etc).
Why does it need to be cross platform? If code was free there would be a native app for each platform using its respective toolkit, not QT.
Qt is LGPL, so free for commercial use in most situations.