This was the unveiling of the film for the general public, not geeks like us who can list fifteen or more cast members.
It could be they figured having Nimoy in it would confuse Joe Moviegoer. Most people know Nimoy as Spock. Seeing him in the trailer with white hair and such may get Joe Moviegoer to think, "Is this a sequel?" He may even wonder if the younger guy in blue with pointy ears is Spock's son.
Also, sadly, having Nimoy in the trailer gives it a link to the past that could be baggage when the goal is to make this seem fresh. Joe Moviegoer could think it's "just another" Trek movie and they just didn't show Shatner and the others. Truly Star Trek XI.