Oprah as a commentator and entertainer is afforded the luxury of being able to criticize, and proposing "big picture" solutions, without ever having to get into the details.
Oprah would get bogged down as having no "solutions" very early on. Her greatest strength is in "rallying troops". That is, she is very effective at endorsing people. But if she were to run herself, she would have to actually stake out very clear positions on a lot of complex issues.... and in staking out specific positions, she would alienate parts of her fan-base who have different positions on those issues.
You would be surprised at how many women think Oprah is "just like them", or "just the way they SHOULD be". If Oprah was forced to stake out positions on hard issues... she would alienate large swaths of her fan-base no matter what.