I believe that's because traditional news media and tv-shows get auto-approved for trending (it's really not trending, it's "recommended"), while everything else (save their top nonsense vloggers, I suppose) has to be manually approved to be allowed to "trend".

They said as much when the "David Hogg Crisis Actor" video was trending. They didn't catch it because the actual video was unaltered content from a major news outlet, so it got automatically approved.