I've wondered this before, but never found much concrete--except that being picky about rendering serifs on "low resolution" screens is a very modern thing in itself!

My intuition is that it comes from books/newspapers and nascent desktop publishing universes; sans-serif fonts being used for large amounts of body text anywhere, on paper or not, is relatively new as a mainstream thing. (That isn't to say it never happened, just that it's historically rare.)