Variety

Hayfever is called hayfever because its season overlaps with haying season. But haying season starts in May in the US. So American hayfever may be prevalent in different season than Japanese one that starts in February.
What is particular to Japanese hayfever is that it is mainly caused by a single species of tree; cedar. Any wind-pollinated plants can cause allergy. (Insect-pollinated plants produce pollens that are too heavy to be airborne, for that matter). In Japan we are exposed to the cedar pollen all the time from February to early May so that the total amount of pollen that we take in our body easily goes beyond its threshold value above which it triggers the allergic reaction in our immune systems. In other countries no plants dominate the other as in the cedars in Japan. Therefore people intake only little of each species of pollen, which makes it difficult to exceed each threshold intake amount.
We can say this boom of hayfever in Japan is the outcome of the fact that we have not been aware of the importance of variety.