At the end of Star Trek Into Darkness, it seems like Carol Marcus (Alice Eve) is poised to join the Enterprise and become an important member of the crew (and break up the near-total sausage party that is the cast). But come Star Trek Beyond, she's nowhere to be found. Now we know why. Image: Screencap from Into Darkness via TrekCore
On the latest episode of the official Star Trek podcast, Simon Pegg answered the question. The real world answer was, basically, that co-writers Pegg and Doug Jung thought that they would "underserve" her character. Pegg also pointed out the fairly obvious point that killing her was a terrible option. No clue why that was something that might have happened, but, yes, it would have been bad.
"We thought rather than have Carol Marcus be not used to a reasonable capacity, let's just not include her, have her be alive, in canon, and be ready to come back at any time," he concluded.
They did try to give a reason for her absence. "I think we had some lines to explain her absence in one of the initial drafts that had her going off to start work on the Genesis project and do sort of very, very early research on that," Pegg went on to explain. "But it didn't make it into the final cut."
Can I just say, as nice it would be to have her back, let's not bring back Carol Marcus in a movie to do the Kelvin universe's take on Genesis. Please.