@Gargron (Have not watched the video) I reckon one very obvious and logical reason is due to very extreme and unnecessary censorship and/or getting monetized has become so much of a chore that a creator sees fit to migrate elsewhere, be it their own website or some other semi-popular website.

I used to make YouTube videos myself, never got popular and part of me is VERY HAPPY about that because I absolutely cannot stand how YouTube, as it is today, is run. In a few words; it's absurd.