I don't think it's ever an easy thing to know why an author does one thing or another. To really know the answer as to why Lorraine Hansberry used a line from a poem by Langston Hughes, we would have to ask her or review a speech that she gave, a letter she wrote, an entry she made in her diary, etc. And even then, all we could be sure of would be that's why she said was the reason.
The idea of an "author's purpose" is a standard taught in high schools used across the United States, but it's a very problematic standard. The New Critics famously argued that the author's purpose is irrelevant to the meaning in a work, and many later theorists have further problematized the idea of the author's purpose.
Now, having said all that, I'll take my shot at answering your question. Please allow me first, though, to rephrase it: ""Why does the line from Langston Hughes' poem make sense to you as the title of the play by Lorraine Hansberry?"
The line from Hughes' poem makes sense to me as a title for the play because the line functions as an allusion. The savvy reader may recognize the reference to Hughes' poem and may remember that the poem is all about "what happens to a dream deferred." Hughes' poem has a list of similes that explore a person's (or a group of people's) possible reactions when their dream fails to become reality. Hughes, like Hansberry, is a famous black writer. Hansberry's play is all about the dreams of African Americans -- to own their own house, to own their own business and make money, to pursue an education and make a difference in the world, etc. -- and the very real obstacles and hardships that interfere with the realization of those dreams.
The smallest of the life forms in the play -- Travis, the potted plant, and the fetus in Ruth's belly -- seem to me to most clearly embody the danger of a dream shriveling up like a raisin in the sun.