In "The Rocking-Horse Winner," why do the father and the mother in the story have too little money?

In D.H. Lawrence's short story, "The Rocking Horse Winner," the problem is not really that the parents have too little money, it's that they perceive that they have too little money. It's a problem of greed. If you don't live within your means, it doesn't matter how much money you make, it won't be enough. We've all seen stories of celebrities and millionaires who file for bankruptcy. It wasn't that they didn't have enough money---they had more than most people would see in a lifetime. The problem was that they spent more than they had. Consider this quote from the story: 

"They lived in a pleasant house, with a garden, and they had discreet servants, and felt themselves superior to anyone in the neighborhood. Although they lived in style, they always felt an anxiety in the house. There was never enough money. The mother had a small income, and the father had a small income, but not nearly enough for the social position they had to keep up." 

Readers see that the family lives quite comfortably in this passage. They have a nice home and even employ some servants. Both parents are bringing in an income, which seems more than enough to meet the needs of the family, but it's not enough to meet the wants of the parents. The parents want more money to continue to maintain their superiority, to send their children to elite schools, and to satisfy their expensive tastes. 
In the opening line of the story, the woman is described as having married for love, but then the love turned to dust. The woman is described as having a cold heart devoid of love for her children. The father is described as someone who always had prospects, but they never amounted to anything. Throughout the story, the house is seen repeating the phrase  "there must be more money, there must be more money."  It is as though the parents believe that money is the answer to all their problems. Money, it is thought, would heal their hearts of stone and make their lives fulfilling. 
When Paul asks his mother why they don't have a car, she answers that it's because his father has no luck. When Paul begins to predict the horse races and gives his winnings to his mother, even those ever-increasing amounts of money are not enough to make her happy.