Other than she doesn’t want it. Is there a reason not to try to convince Michelle Obama? If this is really the most important election in the history of democracy then she should be the candidate. She’d crush Trump even getting in this late.
They will immediately be able to fundraise and build a campaign infrastructure quickly. Big names will come out of the woodwork to work the campaign and donate. She will bring the black, female, and progressive votes back in unity with the party.