Here's why:
* Starter Pokemon: Bulbasaur is one of the three starter Pokémon you choose at the beginning of the game. You don't breed it.
* Breeding Mechanics: In Pokémon Diamond, breeding works by combining two Pokémon of the same egg group. Bulbasaur is in the Monster egg group, but it's a starter and can't be bred.
How to Get a Bulbasaur in Pokémon Diamond:
1. Choose Bulbasaur as your starter: This is the only way to get a Bulbasaur in the game.
2. Trade: You can trade with someone who has a Bulbasaur from a different game.
Let me know if you have any other Pokémon breeding questions!