https://www.emarketer.com/content/youtube-adds-thousands-of-ad-supported-tv-shows-free
In order for YouTube to have the largest audience possible, it has to be as General as possible. NC17 and R movies and TV productions would start to limit its viewers by such content categorization.
https://www.youtube.com/howyoutubeworks/policies/community-guidelines/
https://www.shaunpoore.com/r-rated-youtube-content/
https://support.google.com/youtube/answer/7354993?hl=en
https://www.ghacks.net/2017/11/11/how-to-access-age-restricted-content-on-youtube/
https://support.google.com/youtube/answer/2950063?hl=en&ref_topic=9387083
https://en.wikipedia.org/wiki/YouTube#Video_technology
https://en.wikipedia.org/wiki/YouTube#Copyright_issues
Internet_censorship is always a concern, that's why at a certain point a person will have to pay for the kind of data-flow that they want.
https://en.wikipedia.org/wiki/Internet_censorship#Overview
https://en.wikipedia.org/wiki/Internet_censorship#Around_the_world