Because race isn't genetic and isn't scientific. It should competely be removed from the sciences entirely.
Twins with same dad and mom can be different races.
People who have no relation can have the same skin tone and can be labeled as the same race by someone looking at them.
If there are specific genes or biochemicals causing different vitamin status in people, that is not racial, that is genetic and environmental. They need to cite those specific genes or biochemicals to actually conduct proper science. If the mechanism is melanin, then that needs to be properly described as melanin and not race- there are people who racially are black with no melanin - albinism.
Everyone living at the poles has lowered vitamin d status and elevated vitamin A status, that's why you can't eat livers of polar animals or you'll die like those explorers who ate husky liver.
Vitamin D daily amount is super super easy to get, something like 5-15minutes standing outside is all you need. People do more than that when they walk to their car. The reason their vitamin D is low is often due to needing other vitamins and nutrients that work with it. Vitamin D status is closely related to other fat soluble vitamin status (vitamin k, e) and B vitamins and many other things, not just accessibility to sunlight or dairy or melanin content of the skin.
Additionally melanin content of the skin can change a little over time, including with most metal supplementations like copper, iron, and zinc - that's why zinc and copper deficiencies are associated with vitiligo and why giving vitiligo patients zinc can help treat the condition. It's also why skin bleaching works biochemically. It's why you can tan.
What you eat and do affects your melanin content, but it certainly would not change someone's race, because race is an arbitrary grouping of features that includes skin color from various genetic and biological causes, meant to enforce roles and class onto people.
So no, it isn't racial, it is related to vitamin D status.