I think it's just marketing, tbh. Bluesky is exclusive and thus getting into it means You Are Important. Threads is basically one click away for people who already use Instagram, which I guess is their target market. Such is the power of these megacorps, they can just throw advertising at the problem and it mostly works.
I want to hope that eventually people will leave those platforms once the excitement wears off and they realize that they are problematic (training AI with user posts, poor moderation, privacy, etc.). Or maybe some big controversy will happen which causes a Twitter like situation for them. But realistically, I don't see either happening short term.
But honestly, Mastodon has the IRC problem where it seems to be aimed at "techies" and require significant buy-in to its ideology. I made an account with Otaku to follow someone last night, and all it needed was an email and password. Whereas for Mastodon you need to pick an instance, write an application (how much do you need to write? Is it an exam? Will you get rejected?) and wait. The Mastodon team have done an amazing work on making all of this as smooth as possible, but ultimately it's a hurdle that you need to invest time and energy to understanding. And then your friend just sends you a BlueSky invite and you decide to just go there.
I'm not sure I agree with other comments saying that it's because it's too hard or that people think it's too hard... It's more that it's an additional hurdle and complexity that people won't go through unless they agree with the fediverse ideology.
And then there's the elephant in the room, which is probably just my personal experience. My first Lemmy account was on an instance that defederated with a bunch of others, and my "main" Mastadon account is/was with an instance that was silenced by meow.social for an unknown reason. Not a good experience for me. Not to mention the immature "I'll defed you if you don't block Meta!" thing going on right now...
Fear of "Picking the right instance" is legit something I would have left the fediverse for if I wasn't so angry at megacorps all the time. I don't want politics in my social media as a matter of policy, and "fediverse politics" is something that I feel I always need to be aware of. And it results in me making long venty posts like this, because I have Opinions...
Although, I have a question... How many fediverse accounts do people here have? Is having multiple mastodon/lemmy accounts something that's normal, but not really talked about?
EDIT: Wow, I did not realize how much I posted until I sent it. See why I try to avoid drama and politics? :P