So once upon a time there was this social network anyone could host that interoperated with other instances called StatusNet, renamed to GNU Social around the same time it became a GNU project (for a while being a GNU project carried some prestige). Over a few years it gains a cult following in the early-mid 2010s with a small group of nerds online, usually libertarian/free speech leaning, though there was also a socialist left segment of the network as well. This increases after Twitter ban waves and another instance releasing a frontend that mimics Twitter.

Then 2016-7 comes and Eugen decides to start this new social network that's decentralized and while starting a new network from scratch would be an uphill battle, Eugen decides to latch onto the existing GNU Social network using OStatus (while also adding in ActivityPub). Mastodon gets this marketing blitz about how it's "Twitter without the Nazis" from journalists for a month or so in early 2017.
https://www.vice.com/en/article/783akg/mastodon-is-like-twitter-without-nazis-so-why-are-we-not-using-it

This leads to a giant divide between Mastodon instances and the others using GNU Social, like the current one with Lemmy and the rest of the fedi or even the new Twitter refugees. Namely while the GNU Social side was mostly for free speech and similar, the Mastodon side had the Tumblr mob/NPR listener mindset of shutting down everything online. This is why a guide on Mastodon installation will tell you that you must install a blocklist as an example.

But the even larger issue is that now Mastodon has become the IE6 or Chromium of the fediverse. It's the most popular fediverse instance software based solely on name recognition. It had/has so much pull over the network that what Eugen wants, Eugen can get. Instance blocking wasn't a thing before Mastodon and the Mastodon side is still the top abuser of it. On top of that, Mastodon also has increasingly lagged behind Misskey/Pleroma and even forks for features like reacts. Due to this technical and cultural difference, being on the fediverse with Pleroma and Misskey is vastly different.

Aside from Mastodon instances hammering GNU Social instances (thanks to numerous factors including better hardware for Mastodon instances, more of them, and the ways the code was written), Mastodon also basically forced the rest of the fedi off of OStatus and onto ActivityPub by dropping support for the former. GS being abandoned basically also fueled the rise of Pleroma as ex GNU Social users wrote that.

There's a lot more I could get into, I know a few people on shitposter.club (a former GNU Social instance around before Mastodon) know a lot more about this than I ever could because they lived it.