Why Only 3 Star Trek Number Ones Became Enterprise Captain

Lt. James T. Kirk (Paul Wesley) was First Officer of the USS Farragut before he became Captain of the Enterprise.

In Star Trek II: The Wrath of Khan, Spock became Captain of the Enterprise, but only because the Constitution Class starship had been turned into a Starfleet Academy training vessel. Spock gladly ceded the Captain's chair to Admiral Kirk during the crisis when Khan Noonien Singh (Ricardo Montalban) stole the Genesis Device and weaponized it. Spock died saving the Enterprise from Khan but the Vulcan was reinstated in Starfleet following his resurrection.

In Star Trek VI: The Undiscovered Country, Captain Spock became Federation Ambassador to Qo'nos and initiated peace talks with Klingon High Chancellor Gorkon (David Warner). Spock commanded the Enterprise again after Captain Kirk was arrested and framed for Gorkon's assassination. As soon as Spock helped rescue and exonerate his Captain, the Vulcan returned the Enterprise to Kirk, and Spock would not command the Starship Enterprise again.

Worf

Succeeded Captain Jean-Luc Picard

Although two other First Officers preceded Worf (Michael Dorn), it is the Klingon who would succeed Captain Jean-Luc Picard as Captain of the Enterprise. After his years as Star Trek: Deep Space Nine's Strategic Operations Officer and, briefly, Klingon Chancellor and Federation Ambassador to Qo'nos, Worf returned to his role as Security Chief of the USS Enterprise-E in Star Trek: Nemesis. But Commander Will Riker's (Jonathan Frakes) promotion to Captain of the USS Titan and Lt. Commander Data's (Brent Spiner) death slotted Worf into the role of First Officer.

Star Trek: Picard revealed that Worf became Captain of the USS Enterprise-E in 2381 after Picard was promoted to Admiral and charged with leading the Federation's mission to rescue the Romulan people from their sun going supernova. Captain Worf then lost the Enterprise-E under mysterious circumstances that the Klingon claims was not his fault. The details of Worf's tenure as First Officer and Captain of the Enterprise-E haven't been revealed yet, but perhaps Star Trek will tell that story some day.

Why Didn't 3 Other Enterprise Number Ones Become Captain?

Different First Officers Had Different Reasons For Not Becoming Enterprise Captain

Subcommander T'Pol (Jolene Blalock) was the original First Officer of the Enterprise but didn't become her Captain. T'Pol served the dual role of First Officer and Science Officer of the NX-01 Enterprise commanded by Captain Jonathan Archer (Scott Bakula) in the 22nd century, as seen in Star Trek: Enterprise. T'Pol left the Vulcan Science Council to join Starfleet, and the Vulcan was Archer's Number One during the ten years they traveled the galaxy aboard Enterprise. However, there is no evidence T'Pol desired to become Captain of the Enterprise or that she succeeded Archer when he was promoted to Admiral.

Lt. Commander Una Chin-Riley (Majel Barrett Roddenberry, Rebecca Romijn) is literally the Starfleet Officer the term 'Number One' was coined but she also never became Captain of the Enterprise. Una was the First Officer of Captain Christopher Pike in Star Trek's "The Cage" and Star Trek: Strange New Worlds. However, Una does not become Captain of the Enterprise, and James T. Kirk takes over for Pike instead. Why Number One doesn't succeed Pike as Captain of the Federation flagship is a story that will potentially be told in Star Trek: Strange New Worlds.