The Fediverse has lived under three protocols: OpenMicroBlogging (XMPP-based, 2008), OStatus (2009-2017), ActivityPub (2018–).
OStatus was indeed built on something very similar to RSS, it was Atom plus a few more protocols to handle push, comments and other things.
The reason OStatus was abandoned was that the stack of protocols was difficult to implement and difficult to extend. ActivityPub was intended to improve on it, and judging from the number of implementations, it seems it did.
onepict