A Raisin in the Sun

The first act of this play has got me confused. Why wouldn't Ruth and Mama want Walter to take a risk and invest some of that money into a liquor store? My father was a very successful business man and he didn't get that way by being frugal with his money. If Ruth decides to keep the baby that's on the way then Walter is going to have to find a way to convince Mama to let him borrow some money.