Remember when McCoy was freaking Kirk out while hiding in the shadows of Spock's quarters. "Why did you leave me?" "Climb the steps of Mount Selaya." Apparently Spock thought that McCoy would be able to tell Kirk what to do. Something went wrong and in the absence of a written will, Kirk decided for burial in space.
Would Spock not have a written will? Apparently not since nobody knew about the katra, Mount Selaya or any of that.