Even aside from the Poast thing, the truth is most people on the internet don't know what a fediverse is or are merely aware of what a Mastodon is. Let's say instance blocking isn't a thing. The thing is, that person you follow on Twitter isn't on the fediverse and he won't join because his friends won't join.

The hard truth with the fediverse and every single "alternative social media" is that the only way it can ever get big is if it does something the others don't, and in this day and age it's being a containment site or refuge for banned users.

Pawoo was initially big because it tied itself to Pixiv, but also because it was acting as a refuge for banned Twitter users posting loli (or what Twitter jannies assumed was such). Case in point, this post from 2017 from a college creature that in some parts was spot on and others was way off:
https://ethanzuckerman.com/2017/08/18/mastodon-is-big-in-japan-the-reason-why-is-uncomfortable/

"Human rights activists have been worried about intermediary censorship for a long time – I wrote a chapter on the topic for the 2010 book Access Controlled. Decentralized publishing solves some of the problems of intermediary censorship, but not all. As white supremacists are booted from platforms like Twitter and Reddit, they may well seek out decentralized platforms where they set their own rules. (Many have migrated to a platform called Gab, which is not decentralized, but has a set of community guidelines that welcome racist, nationalist speech.) Intermediaries like Domain Name Registrars and Content Delivery Networks may still refuse them service, but neo-Nazis on their own Mastodon server won’t be worried that they’ll be kicked off Twitter, like the Lolicon fans were."

Now let's look 6 years later and who uses the fedi? Not only do both groups use it, but also the writer mentions this:

"Fortunately, there are communities that would greatly benefit from Mastodon: people who’ve grown sick of sexism and harassment on Twitter, but still want the brief, lightweight interaction the site is so good at providing. One of the mysteries of Mastodon is that while many instances were started precisely to provide these alternative spaces, they’ve not grown nearly as fast as those providing space for a subculture banned from Twitter. The Mastodon story so far suggests that sticks may be more powerful than carrots."

They're not the ones sticking around whatsoever, they're the ones closing their instances because someone got butthurt over the word "honk" which led to a drama mountain. It's just so happened that Pawoo stayed up the longest for the same reason why furries air out drama and post art on sites not run by them: Pawoo's administration isn't tied to bullshit fedi drama and has users on the site. These people also don't stay around for long either because gee, it's almost as if the same attention milking tactics don't work on the fedi.

But Poast of all places attracts users despite Graf for one reason and one reason alone: it has the reputation of "Twitter for banned users" because some Twitter user namedropped it in a famous hellthread.