Well, as a critic of the Kelvin-timeline movies let me say: ST09 is a vastly superiour movie to Generations, in every aspect.
Also, the writers of ST09 did the clever thing to lure Nimoy in, and give him lots and lots of interesting material. Sure, he was also the "oldguy exposition machine", like every senior actor in every franchise movie. But they essentially threw all the emotional scenes at him as well, he essentially had more meat to act than in any Trek movie before. That can to wonders to persuade actors.
Wheras on Generations, they wanted him as essentially a glorified extra, basically just standing there while someone else does something, only so they can say he's in the movie. I totally get why he would reject that.