My thoughts are that, early on, Abrams and Co. hadn't decided what they were going to do yet, or who would be willing to participate. It only makes sense that they would meet with both Shatner and Nimoy, both to "keep them in the loop" (i.e., hopefully keep them from bad-mouthing the production) and to see who's willing to participate, should they be needed.
Abrams may have had the attitude of, "We only need one, but let's feel them both out. And if we can get both, even better." Then, as the story evolved, and money and availability were kicked around a little, they likely decided that they only needed one, and that one was Nimoy.
Nimoy likely asked for less money, he makes more sense continuity wise, and he looks far closer to the part of his old character than Shatner does. At that point, they probably thanked Shatner for playing and cut him loose, possibly generating some of the hurt feelings on Shatner's part.
I don't know why it needs to be any more complicated than that. Business, pure and simple.
Abrams may have had the attitude of, "We only need one, but let's feel them both out. And if we can get both, even better." Then, as the story evolved, and money and availability were kicked around a little, they likely decided that they only needed one, and that one was Nimoy.
Nimoy likely asked for less money, he makes more sense continuity wise, and he looks far closer to the part of his old character than Shatner does. At that point, they probably thanked Shatner for playing and cut him loose, possibly generating some of the hurt feelings on Shatner's part.
I don't know why it needs to be any more complicated than that. Business, pure and simple.