Because he doesn’t want to campaign against Obama! Well, I would think this is good news for the McCain camp, the certainly don’t need a campaign adviser with divided loyalties:

A senior adviser to Republican presidential candidate John McCain said on Tuesday that he was stepping down to keep a commitment he made not to campaign against Democrat Barack Obama.
Mark McKinnon, who was in charge of the McCain campaign’s advertising message, said he was still backing the Arizona senator, but that he was simply moving from active campaign participant to cheerleader.