It's mostly mental partly physical.
It's not about hitting a target, but going through the target. Some guys naturally understand that concept while others don't. Some put themselves in a position to hit the target, others put themselves in a position to punch through the target. Speed, leverage, precision, and committing to the punch all play equal part. Too many guys I know are just satisfied with hitting something accurately instead of blowing through it. They're afraid of over committing.
My sifu always told me that guys with strong back muscles have real KO power if they knew how to use it properly.