Now you're overlooking the obvious. Why single Chekov out and punish him for participating in the hijack of the Enterprise? Sulu made Captain, Kirk got a new ship for his demotion (reward), even Scotty made Captain (of Engineering, whatever that is) but poor old Pavel is held back? Sorry, that doesn't make sense.
No need to be defensive. My post wasn't meant as an attack. Calm down.
Scotty had already been promoted to captain of engineering by the time the hijack took place (Morrow's conversation with him at the film's beginning). Unless he was demoted off-screen, he'd already achieved the rank, so there was no need to promote him further. Scotty was never a first-officer, but his expertise in engineering was known throughout the service. His promotion was a reward for his dedication.
Why did Sulu make captain? It's not clear because his promotion didn't happen on screen. I don't think his being promoted was a case of Starfleet holding Chekov back as much as it was rewarding Sulu for his own hard work aboard Enterprise
Again, I think it's likely Chekov's involvement in Kirk's hijack in TSFS played a big role in his not getting a command of his own. But I don't think he was in any way singled out or punished unfairly compared to his shipmates.
One thing that's forgotten about Chekov is how young he was compared to his crewmates (even Sulu, who was his closest friend). Even after serving as first-officer aboard Reliant
, it's possible Starfleet didn't think he was ready for a command of his own, whether it's because of what happened to Reliant
or for reasons we don't know about. He wouldn't be the first person to serve as XO on multiple ships. Riker did it (though he turned down several commands in the process).
Not everyone who serves in the military reaches the rank of captain (or the equivalent). Ranks are earned through a combination of experience and outstanding service. Chekov certainly had the experience, but his performance may not have warranted his getting a ship. Kirk and Spock's opinions would certainly have carried a great deal of weight when it came to his suitability for command. Although both seemed to value him as a member of their crew, that doesn't mean they thought he was leadership material. It's not a conspiracy. It's not because he's Russian. That's just the way it is.