Your hypothesis is falsafiable, and has been falsified. It's conspiratorial thinking, plain and simple. [0], [1]

No, I do not think it is strange that Vit D and NAC are not under a 'huge push'.

The UK made Vit D supplementation available for free last year and the NICE guidelines concluded that there was insufficient evidence at present to support global supplementation [2]. On NAC, there have been a number of small studies but no widespread studies of high quality so neither has made it into regular protocols. That's not to say they won't have some effect, but that the level of evidence to support their widespread usage is not available at present.

Here's what I think is a better question: Why aim for something that either requires high compliance (regular Vit D supplementation, which motivated people can do for cents a day anyway) or pre-intubation Rx (NAC) when you can reduce the risk of hospitalisation (even against delta) or death by 80% with a free, widely available (in some countries), safe vaccine?

It's simply clutching at straws

[0] https://www.the-scientist.com/news-opinion/scientists-revers...

[1] https://www.theguardian.com/world/2021/mar/30/university-sci...

[2] https://www.thelancet.com/journals/landia/article/PIIS2213-8...