Because being aid-de-camp to the guy in charge of latrine/head cleaning details is not exactly getting elected material!!!
I'm not sure if you're joking or not but you're right on target. Kerry was basically the janitor-in-chief on the Gridley in that position. The division he led which included the boatswain mates and those either not striking for any rate or striking for boatswain mates was typically responsible for the cleaning and upkeeping of the decks, main passageways and yes the enlisted mens' head. They also took care of and operated the ship's small boats.
Doesn't exactly fit in with Kerry's Apocalypse Now fantasies.