Most of these boil down to "they might doubt the accusations, and we can't have that."
Plus, they forgot one inconvenient truth about all of this: Moore CAN'T step aside without handing the election to a Democrat. If he did now, he'd still be on the ballot. If he were to be elected and refuse the seat, they'd have to go through yet another round of elections (primary, runoff, general), and nobody wants that. That's why the WaPo waited until now to fish for this story.