Reports of the Undertaker not working SummerSlam came down to money, in addition to his health.
Undertaker realizes at this point in his career he doesn’t have many matches left and couldn’t justify doing a match on a major pay-per-view that was essentially a midcard tag match against The Shield. According to a source, Undertaker wanted “certain financial assurances” on what his payoff would be. When he was unable to get those assurances, he chose to sit this one out.
As of this writing The Undertaker is expected to work WrestleMania XXX next year.
SOURCE - Daily Wrestling News