What do you think causes viral pneumonia?
Clue's in the name. A respiratory virus, of which there are many.
covid-19 variants are generally more contagious than the flu, so mask and social-distancing policies that are somewhat effective at reducing covid transmission are vastly more effective at reducing influenza transmission
Apparently common colds and the like are even more contagious than Covid-19, as the country where I live has had a mask mandate since September 2020 and I have been ill numerous times with regular sniffles and colds.
I take your point with the data you provided (although there's no information on sampling methods and thus the statistical significance of those numbers. We also don't have a similar data point for the UK), but it's a stretch to attribute it to masks and social control measures. I'm not in the US, but I don't think that policies have changed much over the past year - yet flu is back this winter.