Warning: SPOILERS for Star Trek: Strange New Worlds Episode 1 - "Strange New Worlds"
The origin of why Starfleet General Order 1 was named the "Prime Directive" is explained in Star Trek: Strange New Worlds' premiere episode. Set in 2259 in the years before Captain James T. Kirk (William Shatner) takes command of the USS Enterprise, Star Trek: Strange New Worlds charts a new five-year mission of galactic exploration led by Captain Christopher Pike (Anson Mount). However, Pike's first voyage in Strange New Worlds is a rescue mission to save Number One AKA Lt. Commander Una Chin-Riley (Rebecca Romijn) on planet Kiley 279, which becomes Pike's greatest test as to upholding General Order 1.
General Order 1 is Starfleet's fundamental protocol citing non-interference with other species or civilizations. During the NX-01 Enterprise's pioneering exploration of the galaxy in Star Trek: Enterprise, Captain Jonathan Archer mused that someday Starfleet would enact "some sort of a doctrine, something that tells us what we can and can't do out here, should and shouldn't do." Essentially, General Order 1 prohibits Starfleet Officers from using their advanced technology to play god and influence the natural progression of a planet or species. While General Order 1 has been broken several times by many Starfleet Captains, including Pike in Strange New Worlds, there's a reason why the law is referred to as "the Prime Directive," and that reason is finally revealed in Strange New Worlds' first episode.
Star Trek: Strange New Worlds' premiere established that Starfleet gave General Order 1 the designation of the "Prime Directive" as a result of Pike's controversial actions on Kiley 279. Admiral Robert April (Adrian Holmes) told Pike, Spock, and Number One that General Order 1 has been renamed the "Prime Directive" to emphasize its importance in the wake of Pike breaking the protocol on Kiley 279 and revealing the USS Enterprise and Earth's history to prevent a world war. Pike amusingly quipped, "That'll never stick," but Star Trek history records the Prime Directive does become the common vernacular for General Order 1.
Interestingly, Strange New Worlds' premiere retcons the Short Trek episode "Q&A." That Short Trek, set in 2254, which is 5 years before Strange New Worlds' premiere, depicted Ensign Spock's arrival on the Enterprise and the Vulcan Science Officer referred to General Order 1 as the Prime Directive. It's possible the producers of Strange New Worlds either forgot or ignored this and any other instance of the name "Prime Directive" being uttered in Star Trek: Discovery seasons 1 and 2. By doing so, Strange New Worlds' premiere establishing the nomenclature "Prime Directive" lets the new series stake its claim at making Star Trek history right from the starting gate.
Thanks to Strange New Worlds, Starfleet General Order 1 officially becomes known as the Prime Directive as of 2259 so that every Star Trek series thereafter can call the protocol by this designation. It's one of the interesting things for Trekkers to look for since Strange New Worlds is a prequel. Another instance of nomenclature in this 23rd-century era is calling a group beaming down to a planet a "Landing Party" instead of an "Away Team," a term coined and became widely used in the 24th-century era starting with Star Trek: The Next Generation. Contrary to Captain Pike's joke in Star Trek: Strange New Worlds, the name Prime Directive does stick and remains even in the 32nd century of Star Trek: Discovery.
Star Trek: Strange New Worlds streams Thursdays on Paramount+.