Because personal accounts are only generalizable when backed by additional sources / data. There's a difference between whether or not this person is being genuine (My bias is yes) and whether or not their accounts are generalizable (My bias is no). Asking for citations is to me a way of asking someone who seems already familiar with what they are describing for more information, so I can learn more / share with others / etc.

Because on HN it's considered poor form to say "I think you're full of shit" or "I don't like what you're saying", instead it's better to say "got a source?". It casts doubt on the truthfulness or provenance of the original poster's assertion, while still looking polite.