By now most fans know that the original cast of Star Trek actually was re-cast and most of the characters were cut from the show after the first pilot. They shot just one episode with that cast, with the lone holdover being Leonard Nimoy as Spock. The rest of the characters would eventually debut on Star Trek, as Gene Roddenberry later repurposed the original pilot into the episode The Menagerie.
The episode uses parts of the then-un-aired pilot as a flashback of sorts to help establish the story. It would be the only episode those actors would ever be in officially.
It wouldn’t be until Star Trek (2009) that the character Captain Pike was formally brought back. It also wouldn’t be until the second season of Star Trek: Discovery that Number One would return, this time played by Rebecca Romijn. Of course, we all know what happened to Spock after the failed pilot, but why was the entire cast sent packing?
Well, the lead for the original series, Jeff Hunter, didn’t like doing television. At the time, T.V. shows had a lot less prestige to them. Falling somewhere between local stage theater and commercials; at least in the eyes of some. So Hunter left because it was beneath him. At least that’s what his wife Joan claimed.
William Shatner and Nimoy told different versions of why Hunter left. Shatner claims that Hunter was fired after his wife Joan* (aka Sandy or Dusty depending on the story) became too involved in the project. Constantly making demands to the point that Roddenberry himself claimed he’d rather deal with a gorilla than her. While much of Shatner’s claim comes second hand and presumably from Roddenberry, the events are backed up by Nimoy.
* There are discrepancies with Joan’s nickname, some calling her Sandy or Dusty.
In fairness to Joan, her story is supported by Herb Solow a major name that helped Star Trek land on NBC, in the book Inside Star Trek: The Real Story. It was also backed up in a letter by Roddenberry that David Alexander put in Roddenberry’s biography. The letter thanked Hunter, and Roddenberry told Hunter that he respected his decesion.
With Hunter out, the network apparently wanted to just redo the whole thing.
That meant Captain Pike was gone. So too were Number One and Spock. Spock was seen as too “satanic” due to his ears, and may not jive with the midwest types, and Number One was played by an actress (Majel Barrett) NBC didn’t have faith in to carry the role. With all of the talent up for a new deal after the failed pilot, they were all let go. That is, save for Nimoy.
Nimoy passed up a few roles, waiting to see what would become of Star Trek, but NBC wasn’t fond of his character. As the story goes, Gene Roddenberry felt he could save either Number One or Spock and told Barrett that he was going to try and save the Spock-character.
Thanks to Roddenberry, Nimoy would sign a new deal, reprise his role as Spock, though with some character changes, and would join Shatner and the rest of the cast for the new pilot.