Two reasons: A. because I essentially learned how to ride a bike only a few months ago, so fatter tires make up for my oft-inept handling, and B. because my only bike at the moment happens to be a hardtail MTB, and although I'd like a more graceful bike sometime in the future, I'm determined to get the most out of what I've got. You can't get that much narrower than 1.35" on 26" wheels anyway, and the added flat resistance and traction over unexpected dirt detours is worth the slight loss of efficiency for me.