What's wrong with the popular definitions? That's just evolution of language, it doesn't make them incorrect.
People on the internet are so obsessed about narcissism, aren't they, what's so interesting about that particular mental illness rather than others?
To answer my own question, I suppose it's because the symptoms (or behavioral manifestations) equate to socially hated traits in a person such as arrogance and selfishness, which is so because humans are social creatures that support each other to survive, rather than being solitary predators.
Because of the nature of the symptoms it makes for a good insult, and also makes people sensitive to the idea of being one. It's an insult because it's hard to sympathize with someone of that nature, unlike other personality disorders; calling someone avoidant, borderline or schizotypal isn't thrown at someone hatefully in the same way, is it?