I think the reason there were so many sitings in the 30s was because it was a pivotal moment in human history, truly a “will this species be good or evil?” moment, and so we got looked in on.
I think that our brains can’t comprehend that sort of technology and that’s why our eyes/phones can’t detect the stuff all the time.
I think it was because cameras dropped in price and increased in convenience enough to be commonplace items owned by regular people. That had a lot to do with the rise of popular outlaws in the same era too, many were self documenting with their new fangled pocket cameras.