A few reasons. First, foaming filaments, especially PLAs, are very, very, very stringy -- you can't really cut off the feed by stopping the extruder, because the small remnant in the heating head will continue to absorb heat and expand out of the head. So having a single continuous path is almost required for decent quality. Second, minimum weight (both for a given strength, and absolutely) is achieved at a single wall; and vase mode is designed to exactly meet both of those constraints.