Why are states having so much trouble finding a humane way of administering the death penalty?

I oppose the death penalty, but if you are going to kill someone it seems that there should be ways of doing it in a fraction of a second, so that any question of pain would be irrelevant. Couldn’t you just blow someone up with a bomb into thousands of separate pieces? You could also have a sychronized robotic firing squad using machine guns. Is there a requirement that the victim’s body has to remain intact?