The difference is that Reddit was still largely usable after the API changes and most affected users weren’t getting banned. Some people have a high tolerance for ads, but a lot of people would find YouTube unusable without an ad blocker, and if YouTube was banning people, they’d be destroying a lot of the inertia against going somewhere else.

If we say that something like 20% of users on YouTube are using adblockers, that’s a big enough addressable market for a competitor to seriously take off. Enough to attract content creators and VC funding to get it off the ground. In the end YouTube would probably win, but it would be after a few years of actually having to compete. The revenue loss over that time period isn’t worth the short term gain of getting some small percentage of users to watch ads.

If YouTube felt like it could get away with banning people using ad blockers and requiring an account to watch, they would have done it a decade ago.