Other people have said some things, but I'll say some things too:
I think it's easy to look at the titans of success and assume it was inevitable, but in the middle were lots of corporate failures. Google failed to beat Facebook with Google Plus, MySpace was massive and is now basically dead, Vine started up and died, Vimeo failed to defeat YouTube, and while I can't quote their names there were other "Facebook but...", "Twitter but...", and "YouTube but .." companies out there that sprung up with VC backing and died irrelevant. Meta currently has threads as an attempt to compete with Twitter, and probably that'll be gone and forgotten soon. So in a sense the fediverse is beating MySpace and Threads and Vine and GooglePlus by having an alternative that's running at all. Not all victories are assured simply by existing or being "better"
A lot of it is Network Effects. People go on PeerTube, there's nothing they want to watch, they leave. Twitter has been around in the tech space for like 20 years, because other tech people at the same conference as you were on it, but most normal people weren't on it until celebrities started signing up. Because otherwise they'd show up on the home page, not see anything they cared about, and then leave. Most platforms these days explicitly prevent interoperability, and the law allows them to, so it's hard to migrate slowly.
The fediverse is explicitly anti-control and anti-centralization. This means it's aggressively and purposely fragmented, which normal people don't care about, but does bring a host of UX problems. Any attempt to paper over these will likely be met with hostility by the existing community and projects, because the solutions to these "problems" tend to involve central authority of some kind or another, and with centralization comes power, control, and attack vectors
The fediverse, similar to above, is pretty anti-profit. That's why it's an alternative to the big popular ones, but it also means it's harder to have solid paid maintainers and disk storage and stuff, compared to something like YouTube or Facebook which are among the most valuable companies on this Earth. It also makes it hard to buy ads, or airtime, or grassroots astroturfing, or celebrity endorsements, etc, which might reach a broader audience and draw people in. That all takes money that the fediverse simply doesn't have.
The fediverse is pretty anti-algorithm, or at least the way the other platforms characterize "The Algorithm" which is to say anti-dark-patterns. We do this both because we are care about our own health as people, so if we're running something for ourselves why would we dark-pattern ourselves, but also because we aren't driven by profit motive, and so usership costs money and gains us kinda nothing, so there's no incentive to "addict" our users. I've seen multiple accounts, and also seen first-hand, people join Pixelfed from Instagram and bounce off pretty quick. And it's not just the network effect, it's also that Instagram and its algorithmic feed is constantly trying to trick you into watching more and more and more. Pixelfed says "what do you want to see?" and the user goes... uh... I don't know. Maybe cats? And Pixelfed says "okay, here's some cats and nothing else. Let me know if there's anything else you want, otherwise bye thanks for coming". Like, people want a firehose of attention, but the platform doesn't want to subject you to that, and doesn't benefit from it, so it goes "that's all I have for you now. Come back later" which other social media never will.
That last one I think is really a big part of it. You have to ask "Why are so many people on Twitter? What do they do there?" and then wonder if them doing that on Mastodon is better? It's obviously philosophically better for them to spend their time on a freedom-respecting platform than a for-profit exploitation machine, but they could also use neither and that would probably be better too. They could go to a library, a cafe, and a park. Is Mastodon essential? Why do you want it to be used by millions and millions of people? What do you get out of it? What does Mastodon get out of it? What do the maintainers of the instances get out of it?
Would the world be a better place if all Twitter users were Mastodon users, or if all Twitter users simply didn't use either?
Also, people today have a Facebook account, and a Twitter account, and a YouTube account, and a Reddit account, and each of those services does a different thing. You mention how there's no events built into Mastodon. Sure, but there's no events built into Twitter, Reddit, or YouTube either. There's no marketplace integration into frendica, but there's no marketplace integration in YouTube either, Reddit doesn't have hashtags, Facebook doesn't have playlists of videos, Twitter doesn't tell you people's birthdays, etc.
Just because different fediverse tools use ActivityPub, it doesn't necessarily mean they all must interoperate. It can be neat sometimes, and sometimes it basically happens by accident, but a lot of the time it just doesn't make sense.
Dogpiled