nyquildotorg, (edited ) I've heard the story about how the A&W 1/3 lb burger at the same price as McDonald's Quarter Pounder failed because America is bad at fractions, but I do not believe that's the whole story.
Incredibly important factors:
A&W burgers sucked.
McDonalds was already an advertising juggernaut by that point. No kids have ever begged their parents to go to A&W to get a burger, which means the parents don't need to buy a bigger burger.