An interesting post on an infrequent topic gets people to Google around for this topic, and to find more postworthy stuff. You won't get this effect with, say,

An interesting post on an infrequent topic gets people to Google around for this topic, and to find more postworthy stuff. You won't get this effect with, say, a new PG essay, because everything loosely related to PG has already been posted to HN the minute it has been published. So their are no low hanging fruits coming in batch as soon as you shed light on the subject.