Deliberate mistake: Richie was an infant in season one, and beginning with this season, he has been aged up to at least 3 years old while none of the other characters have aged up.
Deliberate mistake: When everyone is doing the Urkel Dance on the rooftop, Steve is instructing everyone on how the dance goes since they have never done it before, but there are sections of the dance where he is not telling them what to do, but it is still perfectly choreographed.
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.