Advertisers have a strong preference for the most gullible people, and know where to find them.
IIRC that had a lot to do with WWII footage being plentiful and either cheap or free.
And that literally a whole generation lived through it, and you could find plentiful interviewees to fill time with. I loved watching my grandfather's generation tell stories, because it felt like watching grandpa tell stories.
The pivot to ancient aliens is what the advertisers favor.