Probably because your random internet stranger has never heard of HN.
It's a shame, for instance, that I was ever allowed to find this place.
[1] I'm assuming you'd have some kind of rotating or individualized Project Euler style problems that you have to solve to get in, and that they would be pretty tough. It occurs to me that you could also have temporary student accounts for people who are just getting into the software world as well though.
Invitation based communities are also very self-policing. If you invite someone and they happen to be an idiot then there is egg on your face as well.