Fans of the original series were surprised when Sam Kirk joined the crew of the U.S.S. Enterprise on Star Trek: Strange New Worlds in Season 1. Until then, all anyone had known about the brother of the legendary James T. Kirk was that he was a civilian with a wife and children. With no previous mention of his service in Starfleet, the revelation that Sam had served aboard Enterprise before his brother was astonishing, and left an important question to be answered.
Source：https://www.cbr.com/strange-new-worlds-sam-kirk-leaving-starfleet-reason/
