It's the "now I have to do two things?" obstacle. When the goal is "join Mastodon" and "here's Mastodon" leads you to a common, busy instance with open joining that's a fast path without a lot of extra friction. The "this one is closed, here's a link to a searchable list of lots of Mastodon instances options" instead adds a lot of new steps and cognitive analysis paralysis friction that many people would rather not do.

One of the best techniques I've seen for bringing people to Mastodon is the opposite technique to general purpose servers, such as the approach that is that used by instances like friend.camp and idlethumbs.social where a lot of the users were directly invited by the instance admins (because they were friends in RL, or on other social media) with basically "here's your account, welcome, ask me if you need anything" (or similar). That reduces friction to the bare minimum. It scales appropriately in the decentralized way Mastodon was meant to (as lots of small instances), and that sort of "high touch" of "I'm your friendly neighborhood admin" is the sort of "human scale" that Mastodon can sell a lot better than other social media, when done right, helping differentiate from first impressions why Mastodon isn't just "nerd Twitter".