Here's why:
* Turtwig is a starter Pokemon: You get Turtwig by choosing it as your starter at the beginning of the game. You cannot obtain it through breeding or other means.
* Breeding is limited: While breeding is a common way to obtain Pokemon eggs in the game, it's not possible to breed for starter Pokemon like Turtwig.
To get a Turtwig:
* Choose Turtwig at the start of the game: Your only option is to select Turtwig as your starting Pokemon.
Let me know if you have any other Pokemon Diamond questions!