Clinton lost in 2016. The American people did not choose her, they chose Trump. Yes, Clinton won the popular vote, but the key people, those who live in Florida and Pennsylvania, did not choose her. What about her is going to make them change their minds? Fresh ideas are needed.
The Democratic field is crowded enough by candidates who’s supporters will turn salty when they don’t win already – like what happened with Sanders in 2016 – and they don’t need another. They’re trying to beat Trump, for god’s sake.
Also, she’d not even be entered into the ballot for some states. No matter what way you look at it, at this late, late, stage it makes zero sense for her to run.