Why do all the scenes in "A Raisin in the Sun" take place in the family's house?

Firstly, "A Raisin in the Sun" is not unique in choosing to set the play in an apartment. 

From a technical perspective, it is just far easier to stage a play when all of the action is confined to a particular space. When there are changes in setting between acts, or even scenes, new props and backgrounds may be required. This is very labor intensive. 

However, in this play, the Younger family's confinement to their apartment also serves a symbolic purpose. They are a poor, black family struggling to improve their circumstances. Walter Younger is a chauffeur who wants to open a liquor store (a plan that is spoiled by his dishonest friend and partner, Willy) and put pearls on his wife's neck. Beneatha Younger, Walter's younger sister, is studying to be a doctor. The apartment is typical of what one would have seen in the Chicago projects: it is small, crowded, and likely to become more crowded after Ruth Younger's announcement of a pregnancy. 

The smallness of the space also fosters tension between those in the apartment: Walter snaps at Ruth; Beneatha belittles her older brother for his ignorance; and there is a clash of values between Beneatha and Mama Younger. Hansberry was able to use that tension to explore broader issues that threatened to divide black families and communities: marital discord (arguably the result of Walter's sexism), generational discord, the place of faith in black communities, access to education or the lack thereof, etc.