Well, the 1/3 also had to compete with the double-quarter (aka, half pound, but two patties), which is bigger and feels significantly more substantial as well.
It doesn't help that any place I saw selling them (let's be real here, this is about McD's) was offering an expensive and fancy 1/3rd burger (deluxe bacon, southwest ranch style, Asian sensation with real gold flake, etc) against the old reliable (cheap) quarter-pounder. Perceived value is everything.
I keep seeing people talking as if they didn't exist anymore but they still have about 600 restaurants in the US and 400 elsewhere (not counting the Canadian version which is independent and doing very well as far as I know and actually has more locations than the US chain)
There used to be one near me and there isn't anymore. I really miss the root beer that was on tap there, something they did to it at the restaurant made it special (possibly a different syrup or the way they mixed it).