Here's why:
* Charmander is not a native Pokémon to the Sinnoh region: Charmander, Charmeleon, and Charizard are all Fire-type Pokémon from the Kanto region.
* No in-game method to get them: Pokémon Diamond doesn't include any events or methods to obtain these Kanto starters in the regular game.
Here's how you can get Charizard in Diamond:
1. Trading: The only way to get Charizard in Diamond is through trading with someone who has already obtained it. This can be done via the Wireless Adapter or through online trading websites.
2. Pokémon Bank: If you have a copy of Pokémon Black/White or later, you can transfer a Charizard from those games to Diamond using Pokémon Bank. This requires a paid subscription to Pokémon Bank.
Note: Obtaining a Charizard through these methods can be difficult, as you'll need to find someone who has one to trade or transfer.