Because it was Gen III and they only had ~300 Pokémon, divided into a dozen or so types. Magikarp, Remoraid, and Luvdisc are usually reserved for fishing, as are Gyarados, Barboach, Whiscash and Octillery. Wooper and Quagsire are more marsh Pokémon, not sea Pokémon. Same with Lotad / Lombre. Marill / Azumarill are rabbits — land animals. Corphish is a lobster, which lives at the bottom of the water, so you wouldn't find that while Surfing. Clampearl, Gorebyss, Huntail, and Relicanth are all based on fish that live at the bottom of the sea. The Pokémon that you find regularly while Surfing — Tentacool, Tentacruel, Wailmer, Chinchou, Spheal, Sealeo, Wingull, Pelipper, Carvanha, and Sharpedo are all based on aquatic fauna that lives near the surface.