Per my understanding, the cause for allergies is not fully understood. There was the cleanliness theory which has been debunked.
Do we know for sure that there is real increased instances of allergies in the population ? Or was the same percent of population had allergies in the past but were under diagnosed ?
As far as I’ve been able to determine in a quick online search, the hygiene hypothesis has only been partly debunked:
The sorts of things that exposure to is good for one’s health is more limited than was proposed in the earlier versions of the hypothesis. Some things are good to be exposed to. Some aren’t.
When my mother was a kid in the 1940s, she had a friend who regularly saw a chiropractor for her allergies. Hey, it worked as well as anything they had available at the time.
I do wonder how many of the “frail, sickly children” of past generations actually had some variant of allergies or food or environmental intolerances.