By sticking with traditional fairy tales, it narrows a child's perspective of the world. At such a young age, it is critical for a child to understand that there are people that look, act, and practice different traditions than they do. If a child is only exposed to their own family's perspective and the narrow perspectives of fairy tales, they adopt a closed-minded view of the world.
Fairy tales could instead be written with the same amount of characters that portray different people. In addition, rewriting a fairy tale does not necessarily mean making it more complicated. Even if fairy tales were rewritten, children would understand the basic parts of a story and benefit more from the diversity portrayed.