I think part of the reason is that more is demanded from kids at an earlier age. Kids today get fast-tracked through to college, with more homework, more tests, and more extracurricular activities loaded on them. They are expected to be good at many different things at once and to multitask their many responsibilities. So there's a mixed message that they are supposed to do everything at once, but not rush through it. It creates extremely difficult standards for kids to live up to.

There are probably just as many kids percentage wise that had ADHD in yesteryear, but the standard for being functional in our society has been raised so high that more people are falling below it.