Heck if I know. Maybe more kids go to the doctor and are identified as having it, while the actual percentages haven't really changed.
You know what it is? It's because parents keep their kids in a bubble trying to "protect them from the ills of the air" and they can't build up their immune system! So, they are open to all KINDS of sickness.
Heck, my mom caught ME eating DIRT one day. Boy, did I get it. But it sure didn't hurt me!