It’s the explanation given by even liberal news outlets. Image

The full source from CNN

I’ll concede that they didn’t know the specific mechanisms of the virus (hence hand sanitizer despite it mostly doing nothing), but it was pretty clearly respiratory (given its close relatives SARS and MERS) and, well, masks work against that. Also to clarify my position so you don’t get the wrong idea, I’m radically pro-mask and think that repealing the mandates was and is social murder.