Ben Carson already has a high platform. Running for President could hurt his status. He might be this year's Presidential election's Sarah Palin, where he goes around America (maybe a bus tour!) and says he's really really considering running ... and then doesn't.
Sarah Palin might be this election's Sarah Palin. Donald Trump routinely does this as well.
If you're asking "But couldn't Ben Carson win!" No, not a chance. Only three men have been President without holding any elected office, and all 3 were war heroes. A highly successful surgeon isn't going to get it done, especially with an increasing list of poor comments.
I was trying to think of someone that hasn't held office before that would have a sliver of chance of winning. Billionaires like Bill Gates or Mark Zuckerberg (when he's old enough) would be intriguing, but what billionaire would want to waste their time on the Presidency? Ross Perot did have a chance by running as an independent. However, any analysis from him might be null by now with people becoming more partisan than ever.
Colin Powell had a chance, but never ran. David Petreus as well, but he declined.
All roads lead back to Oprah. Widely respected, very low chance of any unknowns coming out that would effect her. Democrats would dump Hillary in a hot second. Great communicator. Personable.