Well, firstly the team still has to port Krita to Qt 6. That's not trivial because Qt 6 dropped support for Angle and Krita relied on it (especially on Windows where OpenGL drivers aren't the best). So they will need to figure that one out (last I heard they were thinking of manually patching Qt 6 to support Angle again).

Secondarily, Qt 6 still has quite a few bugs (e.g. related to input) that will need fixing or patching so that Krita can use it.