Vets have been using injectable PPS (with great results) for...

Vets have been using injectable PPS (with great results) for maybe 20 years and have been scratching their heads as to why the stuff hasn't become a similar, but much larger scale, success story in the human market. There has long been at least 3 products (I thought manufacturers) to choose from, the drug being marketed variously as Cartrophen Vet, Pentosan and Zydax.
How does this plethora of options square with PAR having exclusive rights for 20 years from the only manufacturer in the world? I know nothing about how such things work and, although I have been waiting to jump on the bandwagon, it does make me somewhat sceptical.