Because both Orion and Ariel would be monstrous infantry or straight-out monsters. Clamshell packs are one thing. I can't remember the last time they came out with a plastic kit for a special character that wasn't combined with something else (Durthu being a prime example of special character combined with plastic kit). Since Ariel and Orion are unique in their appearances I doubt we will ever see them in plastic since they really don't share an aesthetic with anything else in the WE army.