There are other projects that support the client to server part of the specification (see my profile for an example in the FedBOX project, and from the more mainstream ones there's Pleroma AFAIR). Generally developers justify not adding it to their implementations with the fact that is not specified in sufficient detail. Personally I think it's mostly laziness and a lack of imagination - but I'm biased.
I'm toying around with my own and that's interesting. I'd love to support it but if no one else supports it no clients will support it either. Granted eventually i expect i'd want some client-server API so i may as well use ActivityPubs. Nonetheless the lack of adoption feels odd.