The reason is simple: all the main symptoms of narcissistic personality disorder involve behavior which is harmful to others.

In this, it is different from most forms of neurodivergence. Like psychopathy, it is a kind of antisocial personality disorder, in the sense that people who have it are harmful to society. Autism, ADHD, bipolar, BPD and even schizophrenia do not necessarily cause the sufferer to become harmful to others - in fact, the person afflicted with these conditions often harms themselves on an attempt not to inconvenience others, and only harms others unintentionally, while the narcissist does so intentionally and deliberately. People don’t demonize narcissists. Narcissism demonizes narcissists, and people recognize that.