VTK users don’t care what underlying technology (OpenGL, OpenGLES, Vulkan, Metal, etc.) should be used to create a VTK render window in a Qt widget. Would you really want to create #ifdefs around instantiation of the render window in all Qt VTK tests, examples, and applications?
If you want, you can still expose specialized classes that allow users to choose a specific technology, but the QVTKRenderWidget should just work, with the best underlying available technology automatically selected.