Here's why:
* Surf is a HM (Hidden Machine) move: This means it's learned by using a specific item called a TM or HM.
* Poliwag doesn't learn Surf from any TM or HM: While it can learn other water-type moves, Surf is not on its move list.
Even though Poliwag is a water-type Pokemon, it doesn't have the ability to learn Surf in Pokemon Crystal.