Why is Mama so set against opening a liquor store in A Raisin in the Sun?
The answer to this question can be found in Act I scene 1 of this excellent play. When Ruth speaks to Mama about the money she is going to receive as Walter asked her too, Mama gives two reasons why she is not willing to use the money which is legally hers to invest in a liquor store. Firstly, she says that the Youngers are not "business people," and thus they are not going to start being business people straight away. To invest so much money in something that you have no experience of is obviously a big deal, and Mama is concerned about this aspect.
Secondly, however, she is also uncomfortable with it being a liquor store, and she is clearly unhappy about her money being used to sell something that she personally doesn't agree with. Note what she says to Ruth:
Well--whether they drinks it or not ain't none of my business. But whether I go into business selling it to 'em is, and I don't want that on my ledger this late in life.
Thus we can see that Mama also disagrees with drink, and she doesn't want to use her money to do something that she disagrees with.
Mama is deeply religious (as demonstrated by her heavy-handed philosophical clashes with Beneatha), and she looks down upon drinking as a vice that makes one focus on the pleasures of the here-and-now, as opposed to living one's life as ascetically as one can in order to reach the gates of heaven. Also, Mama has seen how alcohol has negatively affected the poor people in her neighborhood, one of whom lives right in her apartment---her son Walter. She refuses to play any part in his scheme, as it will mean that she will be a contributor to the destruction of her own people.