Reasons for the increase in ADHD diagnosis

Many people seek diagnoses (this is true for ADHD and other diagnoses). Researchers seek diagnoses to help them organize their investigations of etiologies and treatments. Drug companies seek diagnoses so that they can coordinate medicinal treatments with diagnoses. Schools seek diagnoses to justify extra services and to justify achievement scores that do not meet performance expectations. Insurance companies seek diagnoses to substantiate medical necessity.  Individuals seek diagnoses to justify failures, to access accommodations and services, and to direct them to particular forms of treatment. And doctors (and other practitioners) seek diagnoses so they can recommend appropriate therapies.

People who struggle want to know why they struggle and they want a remedy. The diagnosis is supposed to provide the "why" and the hope of a "remedy". However, with ADHD (and with other psychiatric disorders) the diagnosis is a "category name" for a list of behaviors (not an explanation) and the recommended "treatments" that coordinate with the category name have been less than stellar (particularly in the longer term).

Since a diagnosis justifies access to particular medications and services (and those have the potential to increase performance), of course there will be many people claiming the diagnosis as it will give them access to the performance enhancements. And the more people hear and talk about a diagnosis, the more they see it.