A baby may not sleep in a crib because a crib represents a change in routine. A baby may become accustomed to falling asleep during or after feeding, which is not the case when she is set down in a crib.
A crib may be an unfamiliar place for a baby as she doesn't spend any time there when she's not trying to go to sleep. It may be beneficial to let a baby play in the crib during the day to develop more familiarity with it. It may also help to place toys in the crib to reduce a baby's resistance to being there and make her more willing to go there when it's time to sleep.