Mastodon and ActivityPub in general have UX issues. I can't imagine trying to explain to non-technical users why they have to open a pop-up window to another website just to be able to follow someone, or why people keep changing handles, etc etc.

For most people using social media outside of the context of a professional organization, it also means trusting whoever operates the instance, most of which will probably be less reliable and potentially trustworthy than Twitter was or currently is under Musk ownership.