Star Trek: The Next Generation had some notable guest stars over its seven season run, including Dwayne Johnson, Teri Hatcher, Mick Fleetwood, and Seth MacFarlane. Star Trek: The Original Series actor DeForest Kelley even put in an appearance in the pilot episode "Encounter at Far Point" before three other iconic characters from The Original Series made their debuts. Mark Lenard [Sarek], Jimmy Doohan [Scotty], and Leonard Nimoy [Spock] were all part of the series, with Nimoy in a two-part episode.
Not all of the actors on The Next Generation had the opportunity to work with the actors from The Original Series, and for Marina Sirtis, she was very hurt, according to an interview in the August/September 1997 edition of Star Trek Communication, The Magazine of The Official Star Trek Fan Club, that she never got to work with Leonard Nimoy.
Sirtis filmed a scene with Doohan that was deleted, and she said she "never had any real scenes with Mark Lenard," but to not get the opportunity to work with one of the biggest stars of Star Trek left a mark for the actress. And it's understandable. Who wouldn't want to have that chance?
By the time Unification, the episode in which Nimoy guest-starred in 1991, aired, The Original Series had hit the twenty-five year mark, and Star Trek: The Undiscovered Country, the final film with the original cast, was less than a month away from hitting the big screens. Needless to say, working with Nimoy would have been a big deal.
Sirtis' character, Deanna Troi, wasn't always well-served aboard the Enterprise. Oftentimes, her character showed up for one or two lines or simply appeared on the screen without any lines. Unification could have made up for years of being overlooked had she been able to work with Leonard Nimoy. And it had to have stung that Denise Crosby [Commander Sela], who left the series four years before, was able to work with Nimoy in a different role that gave her more to work with. As ship's counselor and an empath, it would have been appropriate to have Troi break the news of Sarek's death to Spock.
Unfortunately, we don't know why the writers chose to leave Sirtis out of any interaction with Nimoy, but years after the episode air, she was still feeling the sting.