I suspect that, along with better clinical treatments, people getting more sun might be a factor, albeit small. There’s one pre-peer-review paper out showing that vitamin D deficiency is associated with a relative risk increase of about 15% of progressing to an acute stage.

If you look at the NIH NHCS brief on Vitamin D deficiency in the US, only about 67% are ‘sufficient’ (50–125 nmol/l), so we’re talking 15% affecting 33% of the population — so not enough, by far, to explain what’s going on.