I know that the majority of depression treatment (and presumably diagnoses) are made by general practitioners, not psychiatrists. At least some portion of ADHD diagnoses are made by non-specialists as well, but I doubt that the proportion is as high as it is for depression, because many of the GP are aware that they don't know much about ADHD and many are leery about prescribing stimulant medication.

But even so, we would have to suggest that there is a recent upswing in diagnosing by general practitioners to explain the recent increase in ADHD.