Edit: I Just thought of a non Gamefreak Logic answer :)
What if Ghastly being poison air itself means it can't learn Poison Gas. I mean, how would it throw gas on it's opponent if it is gas itself? Well it could sneeze I guess, but sneezing, really? It could materialize hands but then what? It can't throw itself. If it blasts/blows itself onto the opponent, what would be the chances of it surviving?
If you've been in the Pokebase for a while then you should probably already know this is the answer. Gamefreak does weird stuff sometimes, like letting Diglett learn Cut, giving Cryogonal attract, somehow a 10 year old had defeated an evil organization and become champion in a span of maybe weeks, and while it is understandable for Gyarados not to be Dragon type so the Dragonite line would be the only ones in Gen I, that doesn't explain why its Mega is Dark type.