I think some parents feel that they are failures if their child isn’t “perfect” and take it personally. So they deny reality and insist that it’s not real, or just a minor problem.
It really sucks that you didn’t get the help you needed, especially since they actually knew. I can understand why mine didn’t get help for me, most people back in the 60s had either not even heard of ADHD or if they did, assumed it was a boy problem.