There was a time that Delta was an also-ran. American Airlines was the leader in the industry, the brightest minds started at American and learned their craft and then went on to run other airlines. Even a mere decade ago it seemed United did its best to copy American, at least with frequent flyer promotions.
Now the unquestioned leader in the US airline industry is Delta. They’re the most profitable and delivering better operational reliability than their largest counterparts. And everyone just assumes they’re smarter.