Seems like DL is relying on hub captives who don’t have other choices or people in competitive cities with lots of competition (say NY or LA) who don’t know what they’re missing by sticking with a US airline.
Most people who are choosing to fly a lot of Delta are flying a a lot of domestic flights, so the international airline comparison isn’t that relevant. And when they do fly international they may be flying Delta largely for the benefits accrued to US flying (i.e. status earning).