Nails are sharp because they have a point with very little surface area. One nail point has a lot less surface area than, say, five or ten nail points like a balloon might encounter on a bed of nails. To pop a balloon, a nail has to exert a lot of pressure on the balloon. Pressure = force / area, so the smaller the area, the more pressure is exerted by the same amount of force. On a bed of nails, the force is distributed over those 68 nail points (remember, more surface area), so the pressure is less and the balloon doesn’t pop as easily. As you saw it took me using a lot of pressure to pop that balloon.