My bet is the licensing stuff which can make it difficult/expensive to make proprietary apps with it. (In practice, I know the picture is a bit complicated.)
Also, personally, while QML probably is the nicest OOP/bindings style UI framework I've seen, it's still not as nice as any of the reactive ones (i.e. React, Vue, Angular2, Flutter, etc…).
Also, personally, while QML probably is the nicest OOP/bindings style UI framework I've seen, it's still not as nice as any of the reactive ones (i.e. React, Vue, Angular2, Flutter, etc…).