Being outside alone isn't enough Vitamin D. In the UK there isn't much sun either.
I would think that there must be something about these individuals, aside from being dark skinned, that makes them more prone to issues.
From https://www.nhs.uk/conditions/vitamins-and-minerals/vitamin-...
They list oily fish, red meat, egg yolks, fortified foods, and liver. I can see some diets not having those.