Most people are pointed to joinmastodon.org first and have to pick an instance. And since they're not familiar with decentralization, they don't understand what that means. It's especially weird that they can't directly join mastodon on the site called "joinmastodon" but have to go to another site.
Then once you get past that to make an account, you have to find people and discovery has always been one of the worst aspects of the fediverse. And the graph of instance blocks means a new user may not even be able to find the people they care about and they won't know why.
If you know all this, its easy to understand. But for people used to a centralized system and unaware of all the intricacies of the network, there's a lot of snags here.