Because they have launched React World, which means from now on if someone makes a react video in a similar fashion to the react series and uploads it to Youtube with intent for it to be monetised they would have to obtain a license from React World and share a cut of the revenue the video makes. They have trademarked the words:

“Kids React,” “Teens React,” “Elders React,” “Adults React,” “React Gaming,” “Do They Know It,” “People Vs Food,” “Lyric Breakdown,” “Try Not to Smile or Laugh,” “Opinions” and “People Vs Technology.” and I’ve even heard that they trademarked the word “React” itself but I don’t know for sure. This means that any monetised video uploaded that uses any of those terms and hasn’t obtained a license from React World will most likely have their video reported by React World for copyright reasons and taken down.

They are literally trying to control and speak for a whole genre of internet content (which they didn’t even invent!) and it’s shit. <3