In a video interview with former Capcom game designer Shinichiro Obata, he stated that the reason why the character Pet Shop is so overpowered to the point of being banned in competitive play is because he was originally intended for the PlayStation version and not arcade one, as such he wasn't really intended for competitive play.

However when the updated arcade version JoJo's Bizarre Adventure: Heritage for the Future was in development, Pet Shop was added as a playable character and Mr. Obata couldn't help but notice how weak this version of the character was. So he started doing everything he could to buff Pet Shop, with Mr. Obata himself remarking that he “might’ve overdone it”.
Contributed by ZpaceJ0ck0