I bet you can find a correlation between the numbers of parents who smoke around young children and the rate of athsma among children.
With more and more parents aware of the negative affects of smoking on children, there are fewer parents who smoke around their kids. So athsma drops.
Maybe there is no true link, but there would be a highly probably link between the correlation and the reality.
There is a link there, it's probably not as strong as some think, but it exists. Asthma is a disease of inflammation in reaction to allergens. So the issue is the level of allergens. The air pollution argument is silly, both because of the drop in outdoor air pollution *and* the increase in the availability of air conditioning, which filters out a lot of allergens.