I’m a wound nurse in a hospital. My explanation won’t be as technical as the excellent one above, but we see nec fasc fairly often (1-2 times a month, give or take), so here’s what I can add.

Nec fasc could occur anywhere, but by far we see it most often in the groin/perineum. It even has its own name in that area - Fournier’s Gangrene.

Now, the trunk is generally well perfused. So is the head, as opposed to the periphery (and especially feet/toes). So why do we see it more there as opposed to in appendages? Because that area has folds that, especially in overweight people, stay moist and warm. Friction between the skin in the folds can create openings for letting in bacteria. Some people are already prone to boils here, which contributes to risk. And some people don’t have the best hygiene. All of this together, and usually with an accompanying complication like diabetes, alcohol dependence, truck drivers (long periods of vibration increasing friction between folds), etc. makes this area of the body ripe for this particular infection.

I have seen nec fasc in arms that had to be amputated. I’ve seen it threaten someone’s life because they dry-shaved right before swimming in a lake (razors cause microabrasions in the skin, lakes are nasty). I’ve seen it in scrotums, around rectums, in labia, abdominal folds, even in breast folds. But I have never seen it in a mouth or throat. Considering how quickly these infections spread, that would be disastrous from the perspective of needing to continue to use those structures for the rest of your life (treatment of nec fasc is generally immediate surgery of a wide area of tissues to contain the infection).