Because Slashdot actually provides the facilities to do so. If I could browse +5 HN comments then believe me I would.
> the populism, trolling, and general lack of real-world experience of other posters made participating in the discussions pointless and even counter-productive.
I could make the same argument about HN.
As others pointed out, even +5 /. browsing leaves you with a stilted half-conversation where you have to expand lower-rated comments to even understand the points being made in the replies. Even worse, often times you'll find some really great comments moderated into the ground there just because they aren't politically correct. Good debate and logical thinking aren't very appreciated there, just snarky comments and closely following the prevailing groupthink.
I could make the same argument about HN.
You could try. I don't think that it would be a particularly good argument. Slashdot's vicious trolling, moderation stalking, and general nastiness is something that I saw way too many times. Discovering HN last year was a huge breath of fresh air. I appreciate it and won't be taking it for granted any time soon.
Your take on /. is also overly negative. I've enjoyed it for many years. The reason I'm here more lately is that I've grown tired of much of the topics there, (such as software licensing) which I have little interest in.