It's down to pure numbers. Biden was the only one of all of them who could draw enough moderates and independents to have a chance against Trump. All the rest would have lost before it ever began. Then biden started a dementia freefall and now they are scrambling for either a new candidate from the convention or a nod nod wink wink vp pick so he can step down after winning for 'health reasons' and the vp is now pres. As to the dementia and its sudden onset I think he is trying to build a dementia defense for the china and ukraine stuff so when he gets knocked out of the race one way or another he can dodge all the questions I'm sure the reformed letter agencies will be wanting to ask him.