Imagine your mom emails you and asks why she can't access her favourite subreddit anymore and you have to explain she now has to make 2 different accounts to access different subreddits. I mean 'instances'. I just picture my own trying to wrap her head around "why".
It sounds like a minor hurdle to our demographic but most people are super lazy and bad at juggling online authentications. I would never recommend Mastodon to any non-tech person for that reason (beyond the other UI issues) unless there was a specific niche community they were interested in.