If you have paying subscribers I'm willing to bet that it's also a lot easier to get demographic data from them (after all, they're willing to pay good money for your efforts), which then makes it (IIRC) much easier to sell higher value targeted advertising. Although possibly the HackerNews demographic is pretty obvious regardless :)

(Almost every news site you go to that relies on ad income will try and get regular visitors to do some kind of survey every year or so for this reason.)