Reverend Mother: Girl groups? Boogie-Woogie on the piano? What were you thinking?
Dolores: I was thinking more like Vegas, you know? Get some butts in the seats.
Biker: If this turns into a nuns' bar, I'm outta here!
Mother Superior, to Deloris: God has brought you here. Take the hint.
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.
It takes 9-12 years to become a nun, with many stages: discernment, aspirancy, postulancy, novitiate, temporary vows, and finally, solemn vows. The young nun may have moved up a step but would not yet have taken her solemn vows. The various stages would have differences in apparel, headdress, insignia, etc. that showed their rank. In addition, different nun orders wear different styles of habits.
raywest ★