The fact that Shatner (I almost typed 'Kirk') was able to move on to other things and redefine his career as he went is a tribute to his ability as an actor.
Nimoy never really got past Spock, and the other original cast have not gotten past those roles either.
I blame Nimoys big time acting faults with type casting. People only saw a scientific alien when it came to Nimoy while the action star Shatner portrayed fell into many roles at the time.
Though I did find Nimoy singing to be catchy.
Peter Jackson got nothing on Nimoy IMO?