They probably couldn't figure out what to do with her. Chekov is kinda just there in the movie too so imagine if they had both characters on the ship traveling to Genesis.
DISPLAY thy breasts, my Julia!

For the same reason why she is barely in any movie, or in any episode. Rodenberry liked to brag about how he put a black woman on the bridge but he failed to explain why she was a totally irrelevant character.

This message has been deleted.

Nichelle does have a singing career, maybe she needed less time in the movie. or maybe she need to LAY down the law with Mr. Adventure

Always thought it would have beamed aboard with Kirk McCoy Scott Sulu and Chekov would have made more sense and given Uhura more of a role.