This was a few years back, and my memory isn’t that great, but from I recall : Diaspora had a rather privileged childhood, in the form of a very successul kickstarter. And they basically were the cool kids back then, and as such they didn’t follow any existing protocol (which, at that time, would have been either OStatus or XMPP, basically) and went their own way. Federation at that time wasn’t that much of a hype, but still they (rightfully) felt it would be great to document their protocol, and they published (some sort of) specification.
At the same time, Friendica’s author (which then went to built several other socialnetworking tools/platforms, as RedMatrix, Huzbilla, Zap, Zot, …) spent some time trying to federate his tools (can’t remember if it was Friendica or RedMatrix) with Diaspora. And was appalled by how unusable the specification was. From what I understood, at least.