I'll see your pet theory and raise you mine: most people are sensitive to the additives added to fortified wheat sold in the States, especially Vitamin B[0] and Iron[1].
I was constantly trying to figure out why people with gluten sensitivities don't have reactions in most European countries. The enrichment theory seems pretty persuasive.
[0]: https://freetheanimal.com/2016/05/enrichment-promotes-everyt...
[1]: https://freetheanimal.com/2015/06/enrichment-theory-everythi...