There are dozens of things being built on the protocol. Just not PDSes. And it’s not because it’s difficult, but because there’s no point until there are others to try and connect to.
Sorry, I feel like I'm not getting my point across: if building independent software that implements a "federated" protocol doesn't give you a "PDS" then the protocol has failed. You shouldn't need a reference implementation to do it, you should be able to federate with multiple instances of your own software or with other independent implementers.