I don't know the answer, or if there is an answer, but I can speculate on a few alternatives.
From what I've heard (but haven't looked into to confirm), incidence of allergies is on the rise. So, allergies may have been more strongly selected against in the past. Prior to antihistamines and modern medicine, if you went into anaphylactic shock, you'd die. So, maybe modern medicine is permitting allergies to increase because people don't die from them anymore.
Then again, if you already had your children before the allergies got that bad, the allergies could still be passed along to the next generation. So, it could be that there isn't enough selection pressure against it because allergies slowly worsen with age, so they don't become life-threatening until you're old enough to have children already.
Or, we may be getting exposed to things ancestral humans were never exposed to, and those chemicals may predispose us to allergies or cause immune system dysfunction.
Maybe we grow up in too clean of environments, so aren't exposed to these allergens early enough in life for our immune system to know how to react to them properly.
Just a lot of possible guesses here.