Why did Sophocles choose not to make Jocasta a major role in this play?

I think
thatdoes occupy a rather significant role in the drama.  Granted, she does not take the same
level of importance thatdoes.  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...