My main question for you is: What is the reason Vitamin D gets relatively little attention compared to the possible benefits of supplementing it?

The main reason I think is that the previous meta-analyses have had mixed results.

Do interests of the pharmaceutical industry possibly play a role here? (Vitamin D is relatively cheap).

It's possible but I think if Vitamin D did become "big" they would come out with a branded version. I think it is more likely that being a vitamin it doesn't get as much attention as other new agents because it is not "glamorous".