Charlie can only get one chocolate bar every year because his family is so poor. They are a family of seven, with four grandparents, two parents, and Charlie. Charlie's father is the only breadwinner in the family, and he works in a factory screwing the caps onto tubes of toothpaste, which barely earns him enough for the family to scrape by. There is certainly no money for luxuries like chocolate.
However, Charlie loves chocolate more than anything, and once a year, on his birthday, his family make the sacrifices needed to buy him one small chocolate bar. Charlie shows remarkable restraint when it comes to these chocolate bars, making them last for more than a month. To understand why Charlie cherishes chocolate so much, it is important to understand what the rest of his diet consists of. Because his father is so poor, the family lives on a diet of bread and margarine for breakfast, boiled potatoes and cabbage for lunch, and cabbage soup for dinner. Not only is this diet dull and monotonous, but it's only on Sundays, when second helpings are allowed, that Charlie as a growing boy gets enough to eat.
It is due to his family's poverty that, despite having to watch his wealthier classmates eat chocolate on a daily basis and living on the threshold of a giant chocolate factory, Charlie only gets to enjoy a chocolate bar once a year.