Big money donors don’t like her. Biden is toast and Bernie’s nuts. Hillary can’t get in if it’s about corruption, so that leaves Moochele and Oprah.
Moochele won’t run unless she is guaranteed a larger victory margin then her husband. And that won’t happen. Moochelle won’t ever want to live with the fact that she lost to President Trump, it would prove her world view wrong and tarnish her image of being very well liked. She wants to keep her ego and image that way.
Oprah, same reason as Moochelle, plus Oprah has Harvey Weinstein all over her as well, that pretty much took her out of the running.