No other protocol has the same mixture of maturity (as you point
out), large existing deployment base, and FSF-assigned code.
RDF? Over a decade of collaboration, specs, libraries, tools etc.
Deployment across Government (US+UK), Facebook, IMDB, Google, Yahoo,
MySpace, NASA etc. etc. Bullet proof royalty free licensing, rather
than patent, non attribution.
For the record, I'm not opposed to the OStatus choice, but I do feel it
slightly inaccurate to say there was nothing else comparable out there.