The first reason is that it is low income. It just has sufficient money to spend on every day. But it doesn't have enough money to saving.
The second reason is that the lack of discretionary spending. Most people spend money to buy or do something for personal contentment. It is unnecessary and can reduce spending. For example, shopping, watching a movie, playing game, etc.
The last reason that family and school don't teach kids about managing money. Lead to they don't know how to save money and make them don't have enough money.