Have you ever gone shopping for clothes with $70 in your wallet, only to find that the things you want cost $150? What happens when you don't have enough money to buy everything you want? You probably don't go home without buying anything. Instead, you choose the things you want or need the most and buy those.
Businesses and the government make the same types of choices as you do. Since they don't have an unlimited amount of money, they have to try to meet as many of their needs and wants as possible with the money and other resources they have available.