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.