So, the short answer is: different platforms were developed at different times with different goals in mind. It just so happened that something OStatus-based (now AP), with a focus on microblogging, became the popular thing. And basically everything else in the space had to build compatibility for the way it did things.

Digging in a bit more, there are a few things that really contributed to Mastodon's success over other efforts at the time:

1. A reasonably polished interface. Many other projects at the time looked worse.
2. A third-party API for clients that was easy to use. Prior to Mastodon, third-party clients absolutely sucked. Additionally, projects like Diaspora didn't have a formal API for years. Most people at the time were just shrugging it off and settling for responsive web layouts for mobile.
3. Word-of-mouth marketing. Mastodon bootstrapped GNU Social's existing community to make something new, and then began getting people from Tumblr and a bunch of other online communities to try it. Early Mastodon was quirky and weird and fun, and the public-square focus of microblogging didn't really hurt the fact that you didn't really know anybody when you got there. You, an early adopter, would just wade on in and be your weird self.

Up until this point, every attempt failed at one of these three things. Diaspora had an okay interface, and great word-of-mouth, but no support for mobile. (They also had other problems in development, but that's another story). Friendica had an ugly interface, a brilliant backend, and a very small hobbyist community. Mike Macgirvin also has a habit of spinning off new projects from old ones, to chase some wild hair he's got regarding how to do something new. So, you have kind of a fragmented community of "kind-of" supported platforms and no marketing.

Anyway, to the point: while the fediverse does have some NIH going on, many of the foundational technologies are shared, like Webfinger and ActvityPub. Contrast this with Tent, which really threw the baby out with the bathwater, and tried to do everything from the ground up with just two guys building it. It didn't end well.

Anyway, sorry, rant over. 😛