Everyone knows the reason, simply because of Vanessa Hudgens' nude photo scandal. From OK! Magazine
Over a month after frisky nude photos of High School Musical starlet Vanessa Hudgens hit the Internet, OK! can exclusively report that Disney has made up its mind about what to do next and that the 18-year-old actress will not be asked to board the boat for the third HSM film.
“Disney finally decided that they don’t want her back,” an insider reveals to OK!. “They feel that as long as Zac Efron is in the movie, all will be fine. He’s the real star — the household name — and, most importantly, he comes without baggage.”
The source also reports that one of the Cheetah Girls — Adrienne Baillon, 23, or possibly 23-year-old Sabrina Bryan — will take Vanessa's place. “The producers think that after Dancing With the Stars, Sabrina will be more famous than Vanessa.”