Here's why:
* Jirachi was not available through Mystery Gift in Pokemon Black. The only way to get Jirachi in Pokemon Black was through the "My Pokémon Ranch" app for the Nintendo DS.
* My Pokémon Ranch was a separate app that allowed players to transfer Pokemon from Pokemon Black/White to the Ranch. You could then send them back to your main game, and there was a chance to receive a Jirachi egg.
* Mystery Gift is a separate feature in Pokemon games used to receive special events and items. It wasn't used to distribute Jirachi in Pokemon Black.
If you're looking to get a Jirachi in Pokemon Black, your only option is to find someone who has one and trade with them. You can also look for online guides on how to use the "My Pokémon Ranch" app to potentially obtain a Jirachi egg.