The people calling for Biden to drop out are supporters of his party, the democrats, who recognize that he’s unfit and incapable of winning. They want him to be replaced by someone else to increase their party’s chances of victory.
No one is calling for trump to drop out because he’s looking more fit in comparison to Biden and he’s projected to win. No one would call for a candidate to drop out when they’re in the lead if they support that candidates party.
Criminal allegations, true or false, don’t enter into it because America has a two tiered justice system where the wealthy and powerful are less beholden to the law than the rest of us and because presidency requires the violation of myriad international laws, norms and human rights.
The job title might as well be War Criminal in Chief and none of the allegations leveled at trump are disqualifying.