One factor is that the people in lower income neighborhoods are more likely to have jobs requiring physical labor, or a job where you're standing all day. After doing a job like that, you have zero motivation to exercise, and you often don't need to to keep healthy. It's also just a fact that lower income workers tend to have longer hours, more stressful jobs, longer commutes, all of which contribute to a lack of motivation at home. It's the same reason people who get on poor people's ass for eating McDonald's instead of cooking all their own meals are full of shit. Cooking is hard work when you're just desperate to relax.