It has already been explained by Abrams himself. Kirk died in Generations, and there is obviously a story element that takes place in a post-Generations timeline (thus the casting of Nimoy as "Old Spock") so there was no way to work "Old Kirk" into the story. You can thank Berman for that one.
And IMO, Trek never rested solely on Kirk. The Original Series was always about the dynamic between Kirk, Spock and McCoy, and the underlying theme being the story about the "human adventure" that just happens to take place in outer space.