It's multi-factorial. They grow up eating inappropriate plant-based foods that damage their guts which causes issues like leaky gut which in turn results in a greater susceptibility toward allergies which clogs up their noses when they're little and so they mouth-breath while they sleep. Top that off with more and more hooyos not breast-feeding for long, giving babies junk like formula, not getting enough animal protein and fat in general (have starches and seed oils instead!), soft and not chewy foods... that's how you go from this about 100 years ago to what we have now:
Somalis I think are perhaps extra susceptible, as are many Arabians from what I've noticed, because a great many of our ancestors (the majority) were still eating majority animal-based diets just 100 years as desert dwelling pastoralists. The average nomad's diet was raw dairy with some meat and extremely occasional wild and cultivated fruit and honey for the sparing time those things were even in season and not nearly as sweet and processed as the honey and fruit available now. Some nomads even went their entire lives without ever consuming a grain by-product like canjeero.
No human is but such people are probably especially not well-adapted to be guzzling breakfast cereal, whole-wheat bread, refined carbohydrates and an abundance of vegetables. We're like Pottenger's cats at this point:
But it can be similarly reversed within a few generations of eating
our species appropriate diet.
development as well. 
I knew I wasn’t crazy
