What pushed YouTube over the edge was it had backers that were willing to overlook and fight for the rampant copyright violations that it had at the time. Then once big enough it then did the deals to go legit, which it now enthusiastically supports for any new entrants.

No other streaming site would have got away with that. Napster was also a bit of a demonstration of how it probably wouldn't work, so it wasn't a low risk strategy.