People have gotten used to centralization. With most current social media, you download one app, create an account, and start following. So when people try, say, Mastodon, they expect it to be a Twitter clone. Then they have to deal with things like instances and 8t goes counter to their expectations.

I think that over time, development of more beginner-friendly clients will smooth out the expectation vs reality disconnect newcomers experience, as the advantages of decentralization become clear.