Why is Raisin in the Sun so long?
Considering the number of characters and related "subplots" of the various family members, the play is actually quite compact and not overly long, in my opinion. My edition is 146 pages long, and considering that it is a play, the page format of the dialogue leaves a lot of space between the characters and their speeches. If you are frustrated with the length, then consider what you think is the essential plot line and then ask yourself what the sub-plots have to do with the "main line" of the story.
Clearly, Mama's plan to use the insurance money is in direct conflict with Walter's dreams of one day buying a liquor store and being his own boss. This is the central plot line. All of the text that is related directly to this is essential. That would include all the conversations about the liquor store and why Walter wants to do it; Mama's compromise with regards to how the money is going to spent; Walter's conversations with his friends whom he plans to go into business with; Walter's handling of the loss of the money; Walter's conversation with Mr. Lindener about taking (or later not taking) the money his association is offering the Younger's to not move to the new house. This plot line is about family, dreams, being an individual, and having pride. It is enough to make a compelling story, but my adding a significant sub-plot about Walter's sister Beneatha and her dreams and eventual individuality, the reader gets a broader and more varied picture of those themes. Walter and Beneatha both want a form of the American Dream, but Beneatha's dreams are grounded in her education and her goal of being a doctor -- a very different dream than the much seedier dream of owning a liquor store of all things. Beneatha wants to embrace her African heritage, while Walter thinks of his heritage as a hindrance and something to try to put aside. Walter's goal is wealth and Beneatha turns down the wealthy possible boyfriend, George, in favor of Asagai who inspires her more emotionally and intellectually. That Walter loses her portion of the money as well as his own ties the two plot lines together.
While the play might not need Beneatha and the extra length her story line adds, that plot line does contribute another layer to all of the themes that Hansbury is establishing in this fine play.