It's not that simple. I never said speed was not part of the equation. Yes there has to be a certain amount of speed, speed is important. Without the right form you won't get concussive power. By your logic Malignaggi punches harder than Juan urango, and Chris Byrd hits harder than George Foreman.
Guts, most feather fisted fighters like to punch and get on out. Sometimes in order to punch with power you need to sit down and throw, it opens you up to getting countered and possibly knocked out. Some guys would rather pitty pat and pot shot their way to a UD than take a chance and get knocked out.
It's almost all technique (which includes distancing, timing, mental focus as well as the physical motion) Weedy little guys (or even girls) can learn to break bricks with a punch. But you would be able to benchpress 2 or 3 times as much as they can. Of course, with a strong person and a weak person both throwing with exactly the same technique... the stronger person will land the more devastating blow. Strength is a factor, but it is far, far outweighed by technique. Technique isn't genetic. The brains and co-ordination to learn technique is partly genetic.
A lot has to do with the size of the fist. Malignaggi has hands like a girl. Floyd also has small hands. Guys like Mayorga, Corrales, Foreman, McClellan, Jackson ect had HUGE hands.