If they aren't in poverty you have more options for reducing pollution.
When someone is so poor that they can just barely feed themselves and their family, you can't realistically expect them to do much to reduce pollution. You can't really enforce pollution regulation because they can't pay fines and tossing them in prison only makes things worse.
When they're able to afford some luxuries they're capable of devoting more thought towards their environment and can afford to spend more for more environmentally friendly living.
When someone is so poor that they can just barely feed themselves and their family, you can't realistically expect them to do much to reduce pollution. You can't really enforce pollution regulation because they can't pay fines and tossing them in prison only makes things worse.
When they're able to afford some luxuries they're capable of devoting more thought towards their environment and can afford to spend more for more environmentally friendly living.