Once you get big you change your priorities. There is no reason the same can't happen to hacker news either. If they start allowing low quality shitpost. Right now I assume with the amount of traffic here, it doesn't cost lot of money to maintain this site. But the same for Reddit is different.

Reddit on its barely makes any profit and had made losses for a number of years. Right now they are too big to pull Digg and not suffer any major consequences.