To his credit, Biden adhered to the give/take of ideas with Trump. It’s courteous to let the other person speak and I’m sure he thought those watching would clearly see he’s the reasonable person in the debate. Since Trump has no such courtesy, he just ran out the clock with nonsense, making Biden look weak/confused rather than reasonable. So they wanted the mics cut. I think they’re confident in Harris’ ability to put Trump on his ass when he tries with her.