The study of Star Trek's Vulcans
According to his second autobiography, Mr. Nimoy turned down the chance to appear in Phase II because he was offered only TWO out of eleven episodes, and he felt it was an insult -- to both himself and to Spock -- that Spock was seen as so unimportant that he would only appear in 18% of the episodes. This is different from not being interested! That suggests to me that he was deliberately offered an insulting contract in order to get him to leave, so that someone else could be hired. Nimoy and Roddenberry often butted heads about Spock, with Nimoy wanting to preserve Spock's goodness and dignity; I think Roddenberry may have wanted an actor who would be less protective of the character.