What really gets to me is why Sulu even got the Excelsior in the first place BTS: George Takei requested that Sulu would have his own ship in the final movie
Takei requested nothing of the kind. Read his autobiography. He was
stunned to open his script of ST VI to see that Sulu finally got his long-promised promotion. Takei was the last regular to sign up for ST II. Mere hours before principal photography started. He had no intention of doing it if all Sulu got to do was press some buttons at the helm console again. Harve Bennett explained that the story was pretty much settled, but what about a scene where Kirk tells Sulu about his upcoming promotion to captain? (The line used mentioned a ship called Excelsior, and Vonda McIntyre elaborated on this in her three novelizations; indeed, it actually matched up with her Sulu subplot in "The Entropy Effect", which is where Sulu's first name was coined). Takei agreed, and the story goes that Shatner deliberately fluffed his lines in each take so that the footage wasn't usable.
Takei also held out for better scenes in ST IV, and they gave him more bait: the meeting-his-own-grandfather scene, which was then spoiled when the child actor threw a tantrum and the light started to fade over San Francisco. They gave Sulu the Excelsior in ST VI because it had become a running gag in fandom (and the three McIntyre novelisations) that he was the rightful captain of that ship.
Shatner liked to put his character first and would often take take other characters storylines and/or write out other actors lines that make characters sound objective
Yvonne Craig (Marta in TOS) once told me that Shatner had lots of tricks to keep
his face on camera and not the guest star's, including rolling his eyes to (jokingly) spoil their closeups. Supposedly all in fun, but not very fair.
In Universe: Why Sulu over Chekov or Uhura? Uhura as a Commander was on track to First-Officer and eventually Captain
Sulu and Uhura were always the same ranks in TOS (lieutenant), got the same advances in TMP (lieutenant commander), and ditto ST II (commander). But Sulu wore command gold in TOS, and Uhura wore red. Sulu was command track.
Chekov in TMP was LT in comparison to Uhura and Sulu's LT Commander rank, prior to TWOK he must have accomplished a heck of a lot, in order to match their rank and overtake them on assignment
As of ST II, Uhura was Admiral Kirk's administrative executive assistant at the Academy (publicity notes). She probably ran his office
and taught classes. (Note how she ends up holding Kirk's birthday gift from Spock.) And, according to the cut Sulu scene,
he had just been promoted to
captain of Excelsior. These are worthy posts; Reliant was a science vessel exploring boring backwater planets. Maybe it wasn't such a plum assignment? It was only the events of ST II and III that prevented Sulu from accepting his promotion.
I can understand that the Enterprise crew worked so well with and trusted each other that they requested to remain together onboard the Enterprise-A, however after a while, especially since Kirk was planning to retire, the Officers started to think about their careers
Well, to give everyone their own starship, and have Admiral Kirk commanding their squadron, isn't very practical, nor satisfying for audiences.