I have no idea but I think it's just because of the difference in the way modern mothers think compared to mothers in the 90s and stuff. Modern mothers do not just blindly take medicine and stuff when they are pregnant. Sometimes medicines can have weird effects on the immune system and allergies can be developed when the immune system associates harmless things with disease. This is a wild guess, but I bet many of these people had peanut allergies because their mothers were taking antihistamines or something in the 90s while they were pregnant. It used to be considered safe and often recommended for mother's to take a wide variety of medicine, but our understanding of medicine has advanced quite a bit since then, and we now understand the body as a far more dynamic and self balancing thing, and we weigh the risks of using drugs to be high. In modern times doctors will shy away from prescribing drugs to pregnant mothers unless there is something dangerous or particularly needed.