Because our diet is better than theirs? More of us exercise and pay attention to staying healthy? Take vitamins?
Probably all of that plus sanitation.
My doc back in SFrancisco, head of internal med at UCSF, told me the bacteria, germs, etc in Mexico were worse than Africa. Perhaps the Mexican populace’s collective immune system is just pooped.
I'm pretty sure the average Mexican has the average American beat on exercise, at least in the major cities. Walking is an everyday part of life in Mexico - You walk to work, walk to the bus stop or subway, walk to the market, walk to church, etc., etc.
Traffic congestion, lack of parking spaces and the expense of owning a motor vehicle have all contributed to this phenomenon - and ironically, may also be responsible for the flu's rapid spread in Mexico: While Americans remain relatively isolated from each other in their vehicles, Mexican buses and subways put people in close proximity. The real culprit for this outbreak may be public transportation.