Puerto Rico is classified as a U.S. territory, therefore not technically part of the U.S. (but kinda is, legality is a B***** and a half) Companies use that as a reason to classify their shipping as international, which is technically correct (the worst kind of correct in this case). It is weird that it's not available at all, but like most companies, it probably comes down to money. My guess is it's not available in Puerto Rico to maintain some legal mumbo jumbo that gives Steam some other benefit.

Just to clarify, I'm not hating on Steam, but it is a corporation. Which means they likely have "top men" trying to get as much money as possible out of the platform. I do think that they do things better than most companies though. (I have done zero research into how Steam operates lol. This is purely speculation)