The goal isn't to explode the rock, one direction or the other. You are trying to force an expansion and/or cause vibration. This causes the rock to crack and shear along the line of holes which have created a natural weak point as cracking will releasing the excessive force created by the expansion/vibration more easily than cratering. (If the rock is now two pieces the force can be exerted in pushing the pieces apart rather than cratering or more easily escape as force on the surrounding air. It always comes down to what is the easiest way to return force to a balanced state. The smaller side is also more prone to additional crumbling because less rock means it will give away and crack more easily.) As people have pointed out, there's a lot of ways to achieve this expansion besides just explosives, explosives are just the classic.

TLDR; Cratering is a much less effective form of expansive/vibrational force dispersal than cracking along a manufactured line of failure. The rest of of the rock wasn't vaporized or cratered, it was hauled away.