Here's why:
* Save File Structure: The DS only allows one save file per game cartridge. The save data is stored directly on the cartridge itself.
* Overwriting: If you start a new game, the new save file will overwrite the old one, permanently deleting the old data.
What you can do:
* Second Cartridge: The easiest way is to get a second copy of Pokemon Black. This way, you can have one save file on each cartridge.
* Trade to a Friend: You could trade your old Pokemon to a friend's game, start a new game on your cartridge, and then later trade your Pokemon back.
* Emulation: If you're comfortable with emulators, you can play Pokemon Black on a computer emulator. This allows you to create multiple save files.
Important Note: Emulation involves downloading ROM files and using software that may be illegal depending on your region.
Let me know if you have any other questions!