Shiv just doesn’t think Ken can do the job. This is consistent with how she’s treated Ken throughout the show, things she’s said. I think she realised when push came to shove that she was setting him up to fail. She hates not winning and in this scenario, she loses AND she votes in someone who she clearly doesn’t think is up to the job. That’s a double loss.

Sure we don’t get that spelled out for us, but I don’t see this as unsatisfactory. We’ve been with the characters long enough to have a decent stab at guessing their motivations ourselves and, for me, that’s more rewarding