The US has B52s that are cheaper, but they used B2s for this operation. It seems they don't believe Iran's air defenses to be toothless.

You are correct but although the B52 can technically carry the GBU-57 MOP, but it was only done that way during the initial testing of the weapon. The B2 is the only aircraft the USAF actually uses for that munition in combat scenarios.

Also the B2 is better suited for extreme endurance missions like this where the plane is in flight for >36 hours. It has a toilet, microwave and a cot for the pilots to use during the more mundane parts of the mission.