1. People who are "less well off" (i.e. poor) often can't get a credit card, because they've failed to pay off their balance on credit cards in the past, and their credit is shot.

2. Paying for things in cash is an effective budgeting technique. When you run out of your "spending money" (i.e. money not set aside for bills and necessities), then you can't spend any more, because you literally have no more money in your wallet.

3. Many poor people don't have bank accounts (possibly to avoid overdraft fees, and possibly because they don't trust banks), so they cash all their checks and pay for everything in cash.

4. In general, they tend to use check cashing places instead, feeling that the benefit of having a bank account isn't worth the hassle when they make so little money. & Even honest one-off jobs may pay in cash.

The situation is going to be different for different people, but it's often some combination of the above.