I think YouTube simply has first to market mindshare. I personally use and prefer Vimeo. It's a better experience and quality. I don't think anything technical or special keeps YouTube number one other than first to market mindshare.
It's not fair to Vimeo, because they actually had a copy and YouTube didn't, but because of multiple experiences like that I was surprised to see a positive remark about their quality.
It's the youtube content creators like PewDiPie, Casey Niestat, Kurzgesagt etc that makes Youtube special just like that community that makes Hacker news special.