There are quite a few reasons outsiders may feel the fediverse is doomed to fail:

• The lack of monetization. Without any profit or income (beyond donation), people may feel Mastodon is bound to fail due to lack of funding. I disagree with this, but I can see why people assumes this.
• The confusing (or at least, unorthodox) nature of federation may give the illusion that Mastodon (or Fediverse in general) is too complicated for most people so that no one will use it. While it's not too hard to understand, many people come with the preconceived notion and refuse to try to understand.
• I know at least one big youtuber saying that the name "Mastodon" sounds like a joke.
• The smaller userbase and fewer content. While content quality > content quantity, many people do judge social media by its userbase size, and people feel Mastodon is too small for people to care about.
• The community in Fediverse generally leans more nerdy/fringe. More mainstream content are rarer and nerdy stuff like programming, furry art, or queer content are more common. So there's probably a bit of preconceived notion that only nerdy people use fediverse, while the more general public don't.
• Most prominent people and creator, like celebrities, companies, youtubers, larger game devs, etc haven't moved to the fediverse, so people feel that the fediverse is pointless overall without their and most people's favorite.

Those are the few things I can think of. To be honest, I disagree with most-- if not all of them-- because those factors don't affect my enjoyment here as much and they can be overcome with a little bit more exploration, but I can see how some-- if not all of them-- can be a roadblock to some outsiders from liking the fediverse.