Dream Date - S7-E22
Continuity mistake: Laura says that Steve asked her to be his prom date when they were six years old. In other episodes, she has mentioned that she first met him as a blind date - their date in the first season.
Dream Date - S7-E22
Other mistake: Steve and Laura kiss at the prom, and the moment is never mentioned again. In fact, during the Season Nine episode "Drinking and Jiving", Steve is surprised when Laura kisses him passionately, while she is drunk. He is surprised enough to mention it the following morning and offer to start a relationship (then Laura pretends that she doesn't remember).
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.