Probably because Hygrophila polysperma is on the US Federal Noxious Weeds list. This makes all species of Hygrophila polysperma (which I believe includes that one) prohibited from being shipped across states lines or from being brought into the United States of America. I had forgotten Hygrophila was on the list until I reviewed it a week or so ago.Bump
I'd like to know too. I'm looking for Hygrophila sp. 'Porto Velho' and can't find it (or several other plants) anywhere!