How come Jennifer Martys girlfriend in back to the future 1 isn't the same girl in the second movie

According to the trivia on IMDb, Claudia Wells, the first Jennifer, was unavailable because her mother had just been diagnosed with cancer so she stopped acting for a while. Elisabeth Shue took over the role for the two sequels.

Back to the Future Part II (1989) - Trivia (http://www.imdb.com/title/tt0096874/trivia)

Fun fact: George McFly isn't in Back to the Future III because of a contract dispute. His gravestone can be seen in the first half hour of the movie.