Kate fell in love with a fighter pilot. After knowing him for only a short time, she decided to marry him. Drew dissaproved of the marriage and even went to locking Kate in his basement. However, Kate and the pilot ended up getting married and the two moved to Guam, where the fighter pilot was stationed. Thus, no more Kate on the show.
In reality, the actress who played Kate (Christa Miller) left the show to be on her husband's show "Scrubs."