because her parents/manager wanting too much money, as a result an agreement was made for her to be removed from the series. She was 14 at the time, and hasn't worked in TV since. I'm not sure her absence was ever explained in the series, I think it was just one of those deals where a series comes back in the fall, with unexplained changes.
Many people are asking the question of what happened to Judy on Family Matters. I don't think anyone really knows but recent news reports suggest that Judy left the show in order to take better business opportunities for herself at that time.