This is a limitation of the game's design. It was created with a specific set of character options, and unfortunately, female characters weren't included.
This is something that might feel frustrating for some players, especially since the anime featured strong female characters like Dan's sister, Marucho, and Alice. It would have been great to have the option to play as one of them in the game!
While the original game didn't include female characters, it's worth noting that newer Bakugan games, like Bakugan: Battle Planet, offer more diverse character options, including female characters.