Trivia: In the last scene, when the choir sings at the church, the Blues Brothers can be seen on the background in exactly the same position as the church number in the Blues Brothers film.
Trivia: Bette Midler was originally set to play the part of Deloris actually played by Whoopi Goldberg. The reason they wanted Bette was because of her singing voice and the need for Deloris to be able to sing.
Trivia: "Sister Act" is credited as being written by "Joseph Howard." He wasn't a real person. It was a pseudonym Paul Rudnick made up for the film since his script had been re-written so many times by so many different writers that it didn't resemble what he originally wrote. He settled on the name when "R. Chasuble" and "Goofy" were rejected as possible names.
Answer: It is because she is not, technically, a nun yet. She is a novice, one who is in the "trial period" of becoming a nun, but who has not made the final vows to join the order.
Twotall
She doesn't wear a white cap in the second movie either, and she did her vows.