The be the nasty, cold voice of reason... since snakes can't hear airborne sounds, Parseltongue can't be an ordinary spoken language. It may sound like hissing (oh so sexy hissing!) to the human ear, but there would have to be a magical component to it for it to be understood by serpents. You couldn't just study it like Latin.

It may not be as tricky as the Animagus transformation, but if you aren't born with the gift, it's got to be a difficult process, likely involving spells, rituals, and a very patient test subject. You don't want to accidentally say "Your mother mates out of season" to a fifteen foot reticulated python.