I think the resistance against ADHD as a diagnosis is that the symptoms are basically a description of a slovenly or scatterbrained person. Most people will perceive those traits as moral failings that should be held against people, not something congenital that they can't help.
"If you're not alert, organised and motivated, that's your own fault, you should feel ashamed, and nothing can excuse it," is the basic attitude. "Why, if it was congenital, I couldn't take credit for all my hard work!"