Vitamin D is out of patent so profit margins are limited. Same issue with hydroxychloroquine.
The study is fairly small, so the reduction in deaths (2/50 to 0/25) was not statistically significant. The dramatic reduction in ICU admissions was s/s though. From my perspective the room for doubt on the benefits of vitamin D3 is now very small since this study which was an RCT (randomized trial). I will certainly look at any large RCT if/when it comes in, I am in no state of suspense about this.
Apart from the studies mentioned above, there are numerous other indirect lines of evidence, e.g.:
1. Severity of the disease in migrant communities with dark skin or social mandates of covered skin e.g. Somalians in Sweden, African Americans in the USA. which inhibits D3 production.
2. Death rates in countries with high incidence of vitamin D deficiency e.g. Belgium, Italy, versus those with low levels (Scandinavia even Sweden, who eat oily fish and supplement/fortify).
3. Low impact in countries and communities (e.g. homeless people) with high sun exposure.
There are also very realistic mechanisms and explanations for why and how vitamin D3 would have this effect, and prior studies on the impact of vitamin D3 on respiratory tract infections including other pneumonias.
When looking at the literature in this space, note (as in virtually all areas of medicine) that bad studies abound. Some things to look for: excessively small studies seemingly designed to produce a not s/s result combined with the belief that a non-s/s result == proof there is no result; large intermittent bolus doses used that generate surfeit of D3 then a deficit; excessively small doses; failure to take the D3 with fat to ensure digestion; failure to take vitamin K2 with D3 for optimal results; failure to take into account accumulated deficits and obesity whereby many months or even years of D3 vanish without trace into fat stores; funding sources with vested interests in a certain outcome (e.g. osteoporosis medication suppliers with an interest in a finding that D3 is not useful in treating osteoporosis and you should use their far more expensive product) ...