In a similar instance where one country (the United States) realized that distributing a specific medicine widely to all citizens would vastly improve public health, they discovered that — just as with anti-vaxxers/maskers — people will attribute medical issues that are outside of their control and have no known cause to something that is in their control, whether or not it's related.

"In the past five years, 74 cities have voted to remove fluoride from their drinking water, despite thousands of studies showing it prevents cavities." (2018)

So if the United States government attempted to distribute vitamin D to all people, a vocal minority of those people would respond "over my dead body" and immediately begin a nationwide campaign to prevent that distribution, including requesting emergency court orders to stop it. The US governments aren't typically what I would call "strong of will", and often fold on proven public health steps (such as masks) without trying to put up a fight.

Perhaps it's different in other countries?