We're Going to Disney World (1) - S6-E22
Continuity mistake: In an earlier episode, Steve told Carl that he has to take charter flights because he was banned from all commercial airlines. Yet, in this episode, he takes a commercial airline to Florida.
Answer: It is said that as Steve Urkel became a more popular character on the show, Judy became less important to the storylines, plus either her or her parents were demanding more money, causing her to be removed from the show entirely.