After Star Trek: The Undiscovered Country, no one really knows what transpired in the lives of the original crew of the Enterprise. Some retired. Others went on to work in the underground world like Mr. Spock, but we didn't know what had happened to Montgomery "Scotty" Scott (James Doohan) until the season six episode of Star Trek: The Next Generation, "Relics."
Doohan returns in the episode with his character's life signal having been stored in the USS Jenolan's transporter buffer for seventy-five years. Scotty had been a passenger aboard the Jenolan when the ship got stuck in the Dyson sphere's gravity field. After seventy-five years, Scotty is amazed at the new technology, but he doesn't ask about his friends. And many fans have asked why over the years. As it turns out, one scene actually had Counselor Troi (Marina Sirtis) asking Scotty if he wanted to know what happened with his friends and family. Scotty's response was that he wasn't ready to hear that.
In Star Trek All Good Things: A Next Generation Companion, writer Ronald D. Moore said that he thought the topic would clutter things up. Troi would have to tell him about each of his friends, and that would take a while. On top of that, if Troi had told Scotty what had happened to everyone, it would have essentially been locking the crew into their futures. That would have changed the plot of Star Trek: Generations which released two years later. Though some might not think that was a bad thing, there was no way to know how things were actually going to go for the rest of the Star Trek: The Original Series' characters.
Though only Chekov (Walter Koenig) and Captain Kirk (William Shatner) showed up with Scotty on the bridge of the new Enterprise at the beginning of Star Trek: Generations, the writers/producers had no way of knowing if other actors from the original series would return to the big screen. This might not have been their thought process at first, considering the movie wasn't in production at the time, but they knew Star Trek was going to continue. To do that, it couldn't lock the characters away, although, Generations ended up doing that with Captain Kirk. Hopefully, the captain's fate will be corrected in a future iteration of Star Trek.