I've noticed this, as well. Not just the frequency, but the severity seems to be on the rise. For at least a portion of the increase, I think that education is at the heart. many people who suffered from unknown ailments and mysterious symptoms, often times in virtual silence, are now being diagnosed with allergies, due to the increase in understanding. I also believe that a large part of the increase is due to environmental factors that weren't present, or weren't as widely present, 20 and 30 years ago. People are traveling more, being exposed to a larger variety of foods and environments. It should be expected that this would have an impact.
Thirdly, I do believe there is a psychosomatic factor, as well. As long as the earth has turned, there has been a portion of the population determined to suffer from the latest "affliction di'jeur." With an increase in legitimate allergies, there is also a steady increase in imagined allergies. The mind is a very powerful tool, and is fully capable of mimicking any symptom it needs to, provided that the body is determined to have them. When you put all three factors together, it does explain the increase.