Unanimity is a shield. They can all mutually defer their reasoning for supporting the bill to everyone else who voted, and say that their vote against wouldn't have made a difference. Any vote against would have put pressure on all of the others.

AT&T came up with a deal that pleased everyone, and therefore immunized everyone. There were only eight; it's not hard, and politicians are notoriously cheap.

Isn't such deal called a bribe in normal countries?