One reason is that in fanfiction, Ash can have a more diverse team of Pokémon. This gives him more options in battles and showcases his skills as a trainer in handling different types. Also, he might have access to rare Pokémon that are not commonly seen in the original, making him seem more capable.
In fanfiction, Ash's character can be developed in ways that the original series might not have allowed. He could be more of a natural leader for his Pokémon, inspiring them to perform better. His training methods could be more innovative, like using new environments or special training equipment. Moreover, he could be more confident and self - assured, which would translate into better training and stronger Pokémon. This contrasts with the original Ash who sometimes doubts himself during tough situations.