Why was Kirk promoted to Admiral?

Question

Captain Kirk is a man of action, a bona fide space cowboy who famously made his career pushing the limits of the unknown. He is almost literally in love with his starship, the Enterprise. He has a notorious dislike for regulation and the orders of his superiors.

Everything about his character and history suggest that the ideal place for him to be is on the bridge of a starship, serving as Captain, deep out in uncharted space, where he has maximum autonomy and can best apply his skills.

We also know that there are several characters in Star Trek with similar situations who have declined promotions in order to maintain their positions as Captains and explorers. Most notably, Captain Jean-Luc Picard, as explained in the question which inspired this one.

In Kirk's own words, to Picard:

Don't let them promote you. Don't let them transfer you. Don't let them do anything that takes you off the bridge of that ship, because while you're there, you can make a difference.

However, confoundingly, Kirk himself was promoted from Captain to Rear Admiral in between the events of The Original Series and Star Trek: The Motion Picture.

The question is: why?

Why was Kirk promoted to Rear Admiral given his legendary track record as an explorer, desire for the Enterprise, and dislike for authority?

I think we can all agree in retrospect that it was a terrible decision. He's clearly miserable without the Enterprise and takes her back at the earliest opportunity, then proceeds to waste away until the events of Wrath of Khan, and then is promptly demoted for insubordination.

So, how could this happen?

Answer 1

The Star Trek I novelization sheds some light here (Roddenberry wrote this himself so it is canon)

Kirk speaks via 'holocom' to his former year-long spouse, Starfleet flag officer Vice Admiral Lori Ciana (also revealed to be the second of the two victims in the transporter accident, along with Commander Sonak.)

So Kirk got married for a time to another officer and she promoted to Vice Admiral (it's unclear if this happened during or after). It's likely she influenced him to promote.

The non-canon novel The Lost Years explains that this is indeed the case as Ciana's role is pivotal

While Kirk didn't want to be tied to the Admiralty, Ciana was able to persuade him by promising to do everything possible for Kirk to command the Enterprise again when the refit was completed.

Answer 2

Why would Kirk accept a promotion?

Kirk gives us an answer in Star Trek II, the Wrath of Khan. He seems to be feeling his age and recognises that he no longer has the same vitality he had when he was a younger man. Additionally, the task he was assigned by Starfleet (training command-track candidates) is a plum assignment where he gets to pit his intellectual and strategic capabilities against the best and the brightest that Starfleet has to offer on a daily basis.

McCOY: Admiral? Wouldn't it be easier to just put an experienced crew back on the ship?

KIRK: Galloping about the cosmos is a game for the young, Doctor.

_{^{The Wrath of Khan}}

No doubt Starfleet also gave him some guff about the fact that sharing his skills would help to save their lives and the lives of their crews.

SAAVIK: As I indicated, Admiral, that thought had not occurred to me.

KIRK: Well, now you have something new to think about. Carry on.

---

Why would Starfleet offer him a promotion?

As regards the reason why Starfleet would offer a promotion to such a serial rule-breaker, this is covered in several Expanded Universe novels.

It was in recognition of his sterling work on his 5 year mission.

The rest of our tour of duty was routine; with Sulu’s and Chekov’s help, I was able to time our return to Earth five years to the second after we left. Admiral Nogura, now commander in chief of Starfleet, came aboard with Federation president Bormenus. In a grand ceremony, the entire crew was given medals, I was promoted to admiral, and Decker promoted to captain. We then had a reception on the shuttle bay hangar deck.

_{^{The Autobiography of James T. Kirk}}

They wanted to use his good name after a series of scandals.

Nogura looked back at him. “To be perfectly honest, I didn’t want to come out of retirement, Jim. I did so because I was needed. People knew me, trusted me. My name generated confidence and respect. For that reason, I came back. Your name, Jim, whether you know it or not, generates that same confidence and respect.” He paused to let the words sink in. “I wouldn’t have put your assignment request on hold if I didn’t desperately need you.”

_{^{Star Trek: The Lost Years}}

Ultimately they offered him a roving brief, something sufficient to pique his interest.

A hint of frustration crept into Nogura’s tone. “Dammit, Jim, listen to me. Being a starship captain requires a person to become a hell of a diplomat and think on his or her feet. And you were one hell of a starship captain. What if I made you an admiral without chaining you to that desk? If I made you a special envoy, put you in Admiral Ciana’s department as a troubleshooter?

_{^{Star Trek: The Lost Years}}

Answer 3

I doubt a satisfactory answer could be found in canon. The probable answer is mundane military expedience. Higher rank is THE hallmark of success. There would also be the pressure to make room for younger officers to have their chance at command. No boomer captain wants to give up his sub, but he's gotta make room. Plus, age. You can't be a frontline officer forever. For reference, look at Captain Winters being promoted out of command of Easy Company in "Band of Brothers." He was a fighting officer, a good one, but he was promoted anyway. Easy suffered.