I'm not convinced it's that big a deal, but Dick Morris is:
Oprah sends a message to all American women that it is OK not to vote for Hillary and one to African-Americans that they need to vote for Obama. Were Oprah seen primarily as a black leader, her endorsement of a candidate of her own race running against one of her own gender wouldn’t mean that much. If her reputation were one for putting her race constantly ahead of her gender, her endorsement of Obama would seem automatic. But that is not who Oprah is.
She is iconic to women of all races; to them she’s a woman who is black, not a black who is female. So her refusal to endorse a fellow female seeking the presidency is tremendously significant to women voters. She sends a message by her unusual intervention in a political contest in which a woman is running. It reads: A woman, yes. This woman, no.