20 Things You Didn't Know About Star Trek III: The Search For Spock (1984)

A cut line from Star Trek II: The Wrath of Khan contains this exchange:

KIRK I really must thank you. SULU (embarrassed) I am delighted; any chance to go aboard Enterprise, however briefly, is always an excuse for nostalgia. KIRKI cut your new orders personally. By the end of the month, you'll have your first command: USS EXCELSIOR.SULU Thank you, sir. I've looked forward to this for a long time.

By the Wrath Of Khan, Sulu had been promoted from Lieutenant in the Original Series, Lieutenant Commander in the Motion Picture, to a full Commander. This exchange makes it clear that he is to ascend to the rank of captain, aboard the Excelsior. However, in the Search for Spock, there is no mention of this.

Because of the events of the Wrath of Khan, Fleet Admiral Morrow had Kirk's orders rewritten, placing Captain Styles in charge of the Excelsior, at least while she underwent her trial runs. The novelization further expands on this when it comes to Uhura's participation in the conspiracy.

She remains behind, which is a narrative gap in the film. In the novel, she stays where she is to ensure that any ship-to-ship communication could be jammed, meaning Styles, whom she believed had stolen Sulu's command, couldn't radio for help.