Because people are getting accurate diagnoses and help instead of being put in mental institutes with unbelievably bad conditions or getting yelled at and abused for being "lazy", "difficult" and "too hyper" when in reality it's out of control and a medically proven difference in brain development, proportions, and activity.
Before, people that could hide their symptoms did so out of fear of being outcast or abused, and those who couldn't were traumatized by one of the above methods. It seems that recently the amount of people with ADHD has gone up, but it hasn't. The amount of diagnosed people with ADHD has. Just look at the graph for left-handedness before and after it became socially accepted. It's the same pattern.
All of this can be learned with quick research on reputable sources.
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