When it comes to fast food, the order of importance is pretty much: 1. Consistent, 2. Fast, 3. Cheap, 4. Quality. A&W tried to compete on quality and failed because the quality wasn't there, first of all, and because they failed to be consistent, fast, and cheap, which are more important. McDonald's is the absolute best at #1. In the US, Wendy's is superior to McDonald's for a nationwide chain in quality, but the consistency is more variable, so McDonald's rules. Burger King is faster and cheaper, but the consistency is variable, so McDonald's rules. McDonald's understands the ordering of priorities here, and so they win, it's that simple.
McDonald's understands that not everyone will like McDonald's but that almost nobody finds it outright objectionable and many people love it. As long as they keep doing the same thing in every single store or franchise across the country, McDonald's is always a safe choice for stopping into the drive-thru due to that consistency.
A&W failed with the third-pounder because they had bad marketing and because they failed to understand the marketplace to achieve product-market fit, not because Americans are too dumb to understand fractions. I'm sure there's /some people/ who don't understand fractions, but the vast majority of people I've met understand fractions and use them in their daily life...