It helps shield the detectors from cosmic radiation, shields people living on the ground from radiation produced by the collider, and helps with vibrational iso

It helps shield the detectors from cosmic radiation, shields people living on the ground from radiation produced by the collider, and helps with vibrational isolation. But those are just nice bonuses. The most important reason is that there are already lots of people living a nice small-town life on the surface, and erecting a giant, dangerous ring would wreak havoc. It would require land rights, cut towns in half, split highways, trigger lawsuits, and probably end up costing more money and time than just digging.