Slashdot manages a balance of it. Why can't HN do the same?
While there may be similarities and overlap, Slashdot and HN are different communities and have different expectations and community standards. I don't think it's a matter of HN not managing the balance Slashdot has: it's more that HN's sense of where that balance should be is different. I'm sure there are plenty of members who frequent both sites, just as you do, and are aware and accepting of the differences.
Nobody bothers visiting Slashdot any more, just like nobody bothers with Soylent News. Note that any venerable name on either of those sites essentially doesn't post there any longer. Any accounts you see are either die-hard try-hards or bots/spammers.
While there may be similarities and overlap, Slashdot and HN are different communities and have different expectations and community standards. I don't think it's a matter of HN not managing the balance Slashdot has: it's more that HN's sense of where that balance should be is different. I'm sure there are plenty of members who frequent both sites, just as you do, and are aware and accepting of the differences.