Continuity mistake: When the choir are singing the first song that Deloris helps them with, Sister Mary Robert gains confidence and sings louder. Toward the end of the song, she puts her hands together in a praying gesture while she is singing loudly, but in the next shot her arms are by her side.
Continuity mistake: When Willie and Joey are pointing their guns at Delores in Vince's office, they count to three, waiting for the other to shoot first. In the close-up shot, Willie's gun is closer to Delores' head than it was in the previous wide shot.
Continuity mistake: When Deloris is on TV, we see her lift a stuffed rabbit to try and hide her face, she then gives it to Mary Robert and lifts the kid to her face but when Eddie says "I'm gonna kill her. I'm gonna kill her myself", if you look on the TV, you can see her doing the exact same thing again.
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.