Supposedly, a lot of grain sensitivity issues have been exacerbated by new high-yield wheat strains introduced in the 60s and 70s. But I think it's mostly that glutens simply weren't on most people's radar until recently, and so people simply assumed any resulting health issues were inherent or chronic. After all, everybody is supposed to eat bread, right?

Disclaimer: Strict paleo here, with staggeringly positive results. :)