This card's artwork was changed between the Japanese and English releases. The Japanese artwork (also by Ken Sugimori) features Misty being naked.
There’s been many cases of nudity representing purity (Sailor Moon for example), which is probably what the motive for the card was here. However. Since Misty is a minor, this very well could’ve been taken out of context as child pxrnxgrxphy, especially in cultures where being naked is deemed impure, so the change was made. No one exactly knows why the change was made, but this is most likely the reason.
TL;DR it was probably changed to save face in cultures where nudity isn’t considered pure