Scientists believe that factors such as changes in diet, exposure to pollution, and less exposure to microbes which can change how our immune system reacts to an allergenic threat. It is also thought that allergies and increased sensitivity to foods are probably environmental and related to Western lifestyles.
People who migrate to their adopted country are showing an higher prevalance to food allergies compared to their country of origin this further illustrates the environmental factors.
There is no real explanation why we are becoming more allergic to foods but scientists have a few theories such as improved hygiene so children are not getting so many infections. Infections use the same mechanisms to tackle allergic reactions, with fewer infections to fight the immune turns against things which should be harmless to us.