Great question. This is something that continues to stump researchers, scientists, and public health officials. No one seems to know, but here are some of the things I think may contribute:
(1) Environmental factors, some of which have already been mentioned. These also likely include changes in air quality due to toxic chemicals in our environments, including our heavy reliance on plastics, and heavy use of commercial cleaning supplies.
(2) Exposure issues. I'd have to do more research on this one, but I think the basic idea is that we're not exposing kids to the right things to train their bodies to deal with a range of potential allergens. It probably starts during pregnancy when many women, especially American women, limit their diets, and continues into our hyper-vigilant germ-free environment. Recent recent has suggested that a little dirt is good for these little ones!
(3) Overdiagnosis. I think this is a key factor that not a lot of people talk about. I know from personal experience that it's really hard to stomach the idea of giving your child food that might make them sick (I've seen my child swell up in hives and start having problems breathing), but I think people rely way too heavily on skin/blood tests (which have a huge rate of false positives) to determine their child's allergies, rather than their clinical reactions. I also think probably a lot or allergies are really "sensitivities" and probably come and go.
I guess I have opinions on this!