The exact same reason why only 85%: of the population is fat: DNA.
Your DNA determines the construct of your liver, and some can take more abuse than others.
Eventually, EVERY person's liver succumbs to fatty liver disease, but many have something else go wrong first or get hit by a bus.
Eventually, all peoples will get fat in the same exact way.
It all (obesity, fatty liver disease, diabetes II, etc.) comes down to a matter of time and amount of exposure.
Alcoholic fatty liver disease only differs from non-alcoholic fatty liver disease in that the poison In the first, is alcohol.
In the second, the poison is typically fructose (sugars and most breads/pasta).
You are speaking with the person who probably knows more than most doctors about alcoholism, livers and diet.
I got my start by reading "Fat Chance..." by Dr. Robert Lustig M.D.