A high-level intuitive, if simplistic answer, would be that you want to do batching based on type, and a monolithic scene graph does not arrange things by type;

A high-level intuitive, if simplistic answer, would be that you want to do batching based on type, and a monolithic scene graph does not arrange things by type; nor is it in general a valuable abstraction for rendering graphics, which are ultimately highly non-hierarchical and undermined by the constant need for sophisticated traversal logic.