Because they were all told to do so. The Democrats have a narrative and they used the debate to justify replacing Biden with someone else. Now they're all-in on replacing Biden and anyone who objects will be disinvited from the best DC cocktails parties.
Seriously, after years of coordinated denials that Biden was senile all of the Democrat media flipped their scripts IMMEDIATELY after the debate. No way that wasn't coordinated ahead of time.
It dawns on me, too, that 'performance enhancement' medication is a two-edged sword, and could be used to exacerbate existing conditions to ensure a
bad performance, all according to plan.
Of course, if those performance enhancers had routinely been administered before (on other occasions), merely withdrawing the therapy might have the desired effect, without leaving a trace.