It's almost entirely caused by the "avoid peanuts during pregnancy" self-fulfilling prophecy (if you do so, chances are the next generation will be more allergic to peanuts).
That's interesting. During my wife's first pregnancy we ate peanuts all the time, and lived on peanut butter sandwiches. The kid nearly died when we first fed him straight-up peanut butter, and had to go to the ER by ambulance and give him steroids and epinephrine.
We've now been living in a peanut-free environment for the second child, yet the second child has no peanut allergy.
Obviously blanket statements like yours are ignorant. People develop allergies for various reasons, not just environment, or not just genetics.