One factor that seems to be under-represented in the comments so far is that less affluent people don't have time to run.
As you probably know, endurance running requires a significant time commitment. Anyone training for a marathon needs to run several hours a week. I suspect that affluent people have more leisure time, and that it can be more conveniently arranged to match a runner's schedule.
It also requires a not-insignificant investment in nutrition. Less affluent people in the US already have trouble eating healthy food--training for a marathon could increase the required food intake by about 30%, which would have a meaningful impact on household budgets.
Finally, blue-collar workers are less likely to have pent-up physical energy. It's likely that they've already spent all day on their feet, and would rather invest their leisure time in a less physically demanding activity. White-collar workers have been sitting in a chair all day, and the ones who don't get into endurance sports (or similar) are likely to become obese.
Obviously, all of the above is based purely on wild conjecture, but it doesn't seem implausible.
It's almost 100% cultural. Distance running is the national obsession of Kenya and Ethiopia, two countries not renowned for their economic prosperity. Here in the UK, distance running is strongly associated with the north-east, an area that has always been strongly working class and has been devastated by post-industrial decline. In much of continental Europe, cycling has a similar image to boxing as a tough sport for working-class men to lift themselves out of poverty.
Running is damned near the cheapest sport you can choose to pursue. You need trainers, a road and plenty of carbs. If you can't afford to run, you probably can't afford to do much of anything. American culture has turned it into a bourgeois pursuit, but that's absolutely not the case in many other countries.
Agree that it's largely cultural, but not sure about cause/effect. I suspect that American culture leaves less uninterrupted chunks of time for lower middle-class and below to go running. Anecdotally I get the sense that Kenyan culture gives ~everyone uninterrupted chunks of time to go running.
As you probably know, endurance running requires a significant time commitment. Anyone training for a marathon needs to run several hours a week. I suspect that affluent people have more leisure time, and that it can be more conveniently arranged to match a runner's schedule.
It also requires a not-insignificant investment in nutrition. Less affluent people in the US already have trouble eating healthy food--training for a marathon could increase the required food intake by about 30%, which would have a meaningful impact on household budgets.
Finally, blue-collar workers are less likely to have pent-up physical energy. It's likely that they've already spent all day on their feet, and would rather invest their leisure time in a less physically demanding activity. White-collar workers have been sitting in a chair all day, and the ones who don't get into endurance sports (or similar) are likely to become obese.
Obviously, all of the above is based purely on wild conjecture, but it doesn't seem implausible.