^Then how do you explain the captain of the Saratoga
? In any case, it's important to remember that being a starship captain is something only a small percentage of people can actually do. The idea that everyone must eventually attain the rank because they've hung around long enough goes against the idea of a merit-based promotion system. No question that Uhura was good at her job, but that doesn't mean she'd have made a good commanding officer any more than Chekov would have been.
chain of command was somewhat unusual during the films because of the numerous promotions and demotions that happened to various crew members, but the general pecking order aboard the ship was almost always the same:
Kirk ---> Decker (TMP only) ---> Spock ---> Sulu ---> Chekov ---> Uhura.
I've omitted Scotty in spite of his rank because his responsibilities were confined almost exclusively to engineering during this time period. He was second officer during TOS. Additionally, Saavik often took the conn during TWOK in spite of a lower rank than most of the bridge officers, but I suspect this was decided by Kirk and Spock as a way for her to gain command experience.
Given the above order, Chekov was still far-removed from the center seat. He was Terrell's first officer, but Reliant
was a smaller ship with a less experienced crew: Winston Kyle was at least a Lieutenant Commander and a bridge officer despite being only a transporter operator on the Enterprise
. When he transferred back to Enterprise
after TWOK, he was beneath Sulu in the chain-of-command ("Mr. Sulu, you have the conn. I'll be in my quarters.").