Each of the adult characters in A Raisin in the Sun is burdened with a significant decision. These decisions constitute the drama of the play, in large part, and for this reason we cannot say that there is a single main character in the play.
Walter and Mama are the most likely characters to be considered as the main figures of the play because the decisions that they make can be argued to be the most central to the main conflict of the the play. Walter and Mama are the ones who decide if the family will stay or move into a new house.
Outside of deciding the family's geographical future, Ruth has the power to decide whether or not to keep her baby and Beneatha is engaged in a subtle and importance process in deciding how she will identity herself going forward. Her identity is crucial to the family as well, because in choosing who to be she is also choosing to keep or reject both the love and the values with which her mother has raised her.
This is a play about a family and it's struggles. No family member narrates or receives more than a balanced share of stage-time (or page-time). For this reason, making the argument that there is no single main character is easier and more accurate than making an argument that either Mama or Walter should be viewed as the central figure of the play.
However, again, Mama and Walter are perhaps most significant in their roles in the family with Walter being, formally, the head of the household and with Mama standing as "the source of the family's strength as well as its soul".