Why did Sophocles choose not to make Jocasta a major role in this play?
I think that Jocasta does occupy a rather significant role in the drama. Granted, she does not take the same level of importance that Oedipus does. This might be due to the fact that he is the source of the tragic condition that envelops the drama, and Jocasta is not. Yet, she does possess some implications in the drama that are fairly profound. The first is that she is both Oedipus' mother and wife. This is not fully grasped until the end. Yet, in this frame of reference, her role is significant, for she is both to Oedipus. She does not let on that she is both, but there are moments when she simultaneously appropriates both roles while Oedipus endures internal struggle. Her entrance into the drama is one to reprimand Creon and Oedipus for acting like children, failing to accept the adult standards placed upon them:
Poor foolish men, what wicked din is this?/With Thebes sick to death, is it not shameful/That you should rake some private quarrel up?
She enters the drama in a major way, with profound implications. She speaks as both wife and mother, a blending of roles that is highly significant. For example, in Act III, Jocasta prays to Apollo to restore her husband's/ son's sanity. The offering of prayers for a loved one is something that the reader/ audience can see her do as both wife and mother. Although the former won't be revealed until later, there is a blending of both roles that is essential in this scene. Her importance is magnified at her death, when Oedipus looks at her, for the first time as both son and husband. In this way, Jocasta can be seen as significant and playing a major role in the drama.