For the same reason gligar and gliscor look like poison-types but are ground/flying, why the chansey line and audino’s base form aren’t fairies despite being pink and cute but the snubbull line is, why no member of the porygon line is electric despite being digital in nature, why azurill is normal/fairy instead of water/fairy like its evolutions, why trapinch and vibrava aren’t bug-types, why altaria is a dragon-type, why stunfisk is partially a ground-type and not a water-type despite being a fish, and so on and so forth.
It’s because typing is dependent on what element a pokémon aligns with, not on what it looks like.
The Pokédex According to Bill 