Does anyone know why RC Urinary SO Calm Wet Cat Food isn't on any major retailer's website?

I am trying to order Royal Canin Urinary SO Calm Wet Cat Food for my cat but I can't find it on Chewy, PetSmart, or Petco. Amazon has it but my vet can't approve the prescription for some reason.

I know PetSmart sells it in store but doesn't offer it online. Anybody know of a reason why these major retailers don't sell it online?