Imagine one of those collaborative drawing programs in which multiple users can make marks and the resulting drawing is the sum of all their marks. Should one of the users be able to come back later and claim that he wanted all the marks he'd made deleted, on the grounds that they were "his content?" I'd argue that someone doing that would be violating an implicit social contract with the other users.
Similarly, if someone wanted all their HN comments deleted, they'd ruin other people's comments by making them incomprehensible. And how far would deletion be expected to go? If user y quotes part of a comment by user x, is that also supposed to be deleted if x wants his comments deleted? It should be if it's "his personal data," right?
We do delete individual comments and submissions when people do something they worry will get them in trouble. That combined with the fact that accounts are anonymous and that users can wipe their profiles seems to me to strike the right balance.