Monopolies naturally come from doing a thing the best possible way. YouTube does it. I personally use it very rarely and wouldn't care about its disappearance,

Monopolies naturally come from doing a thing the best possible way. YouTube does it. I personally use it very rarely and wouldn't care about its disappearance, but I don't know of other services serving its users needs better. So: YouTube will cease to be a monopoly if 1) user needs change 2) it stops being the best at serving videos. Until then, it's not mysterious.