It's a sort of chicken-and-egg problem, also similar to the social media critical mass problem.

Creators won't move until the audience is there. Audience won't go until their favourite creators are there. Both won't move until the platform can handle the traffic, but the platform doesn't have the money to afford the required infrastructure until they have revenue coming in from large audiences...

The Pirate Bay is not hosting or delivering video, they are just indexing P2P content and hosting magnet links. Pornhub is closer but not at nearly the same scale as YouTube.