Ping.
We will not explain why.
But they did explain why.
"Upon checking, your channel was demonetized due to re-used content."
Which was confirmed by the poster:"There are a handful of videos that we have received notice about for using a music clip or TV clip that they claimed was not fair usage."
The most important point is it isn't Google causing the problem.
Face it, advertisers don't want their ads to be displayed along side certain content.
Or do you think the advertisers shouldn't have any say on where their ads run?