I don’t know their specific situation, but Walmart is often the only viable option for many lower population areas or late-night needs. A large part of Walmart’s expansion strategy is moving into a small town, then under-pricing all of their stuff at a loss. They keep selling things at a loss until all of the other local shops close, because they can’t compete with Walmart’s lower prices. Then once all of the local competition is run out of town, Walmart jacks their prices up because they’re the only option in town.

They’re also often one of the only grocers that stays open late. This means they’re often the only viable option for third-shift workers, because the alternative would be waking up at their equivalent of 2AM to go grocery shopping.