>>34512711Crawdaunt was made in Water/Dark because it is a brutal crustacean. Nose star, face, and torso pattern slightly resemble a Sharpedo's open gaping mouth, which adds to the scariness.
As to why it's physical, it's because this Pokemon goes in close quarters and beats up its opponents with its pincers. It doesn't fight at a distance. It goes straight in for the kill.
Fortunately Game Freak had the sense to do away with the type-based split in Gen IV and actually achieve a logic-based split instead.
