>>54289435because earthquake needed to be retard strength op in the single player campaign
also because gen 1 was heavily based around bug fighting so a lot of the design was centered around already existing animal traits. traits like big horns, claws, teeth and venom. the only pure poison type mons are a snake, poison gas cloud and pile of toxic waste sludge. the rest are just animals or plants.
the exceptions being the ghosts who needed poison sub typing for balance reasons, and the nidoran lines which are some cool ass poison rabbit dinosaur alien things.