The algorithms got too good.
Now it links POP'S videos to what others who subbed to him watched, but probably doesn't recommend the videos/channels that you are already subbed to (because you've needs those clicks for ad revenue and retention - Watching Kurzgesagt doesn't recommend me VSAUCE because I already watch both regularly, and links me to some other shit).
Facebook has had the same issues, to the extreme that in their ad targeting for users, they had the category "jew hater" - can you imagine what those ads are like?
The Internet has done well to connect us to the world, but also those with questionable ideas to plug themselves in to other freaks who support them. Humans are social creatures, being isolated suggests that how you act or behave is repugnant and encourages you to either suppress it or hide it. Now with the Internet suggesting "Hey freak, come here and speak to these other guys who want a race war", the views become more palatable because you aren't alone in those thoughts anymore.
Take the Bungle off sites for a crazy example. Everyone came from Bungle, a mishmash of everything, and now of the 3 off sites I know of, there's 3 distinct "flavours", Tankie cultists, Progressives, and here as what I can only describe as loosely anarchic as there's all manner of crazy here from different sides of the spectrum. You stay in one of these long enough, you'll align to it more.
This is a sort of large side note. The shooting was awful, I like how NZ are handling it by censoring his face and name online (I've only seen his name once when it broke on the BBC, but I haven't been keeping track, while his face from the video keeps popping up on 4chan threads on unrelated things...). Social media seems to be doing a decent job taking down the videos before they spread, but of course there's not much you can do to stop determined madmen from spreading their shit around and getting other nutters to watch it.