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Because it had the bad luck of being the Grass-type starter on a region were 5 of the 8 Gym Leaders has a type advantage or resist Grass, and only beats 2 Gym Leaders (1 of them being the Ice-type Gym Leader which the Chikorita line is weak to), the Team Rocket which almost everybody use Poison-type Pokémon, not a Female only species despite its appearance, not gaining a new typing to help it against the Gym Leaders of Johto, etc.
At least it's a good companion on Mystery Dungeon.