It's not. It's genetic. Heritability for autism is higher than for any childhood disorder that isn't caused by a single gene. There are environmental factors, but they're minor compared to the impact of genetics. (For example: Congenital rubella syndrome raises the risk of autism. Moral: Get your vaccine before you get pregnant. Rubella during pregnancy is no joke.)

Not to say that kids with allergies shouldn't be treated for them. Being autistic is hard enough without having to deal with hives and a stuffy nose too.