I mean of course YouTube is popular. User-generated videos have always been popular (even pre-internet, like home videos on TV etc), but it’s never been the case that storage and bandwidth was cheap enough to not operate a website with videos at a loss.

The only ones being able to operate such a site are entities that have lots of spare cash. Otherwise, if the site gets too popular, it’ll have to shut down or become unusable because of having to limit access behind paywalls or similar, hugely stifling its popularity and likely killing it.

Google created a very good service with YouTube that no one else could compete with because no one had so much money to “burn”. They kept this up for years to a point where it didn’t really make sense for neither creators nor viewers to want to go anywhere else.

And now there is a lot of good content on YouTube. The content is good because creators can actually live off the YouTube payments, thus being able to spend a lot of time on the videos. It will thus stay popular, because creators will not start risking their livelihoods on any other platform.