Because autistic traits aren’t abusive traits? If someone is autistic it doesn’t mean they will be abusive. They aren’t mutually exclusive, but they can overlap.
Whereas narcissistic traits are abusive traits, so a narcissist will almost definitely be abusive. But these aren’t mutually inclusive with each other, so someone can be abusive without being a narcissist.
I don’t know your parents or your situation; if you say they were abusive, I believe you. But if you say they were abusive because of their autism, that is just plainly false. You don’t have to blame it on some mental disorder, diagnosed or not. You can just say they were abusive.