I wager that, for example, most people didn’t vote in california not because they see their candidate as a lost cause, but because they know “their” candidate has carried the state for sure.

That’s a natural interpretation as well. I wonder if it’d be possible to at least guess at whether it was that or “my person won’t win so what’s the point”. There are probably so many other factors. For example the “did not vote map” looks surprisingly similar to the SOVI map: www.atsdr.cdc.gov/…/svi-interactive-map.html