Probably because it's old as hell. I personally read Slashdot every day, simply because I actually like the stories that are presented. I never use site's like Digg because I hate the people and discussions there (hates a strong word, what I mean is that I'm not a fan of the types of stories that are promoted there, or the flamebait comments and "gang up against people of opposing views" mentality that I see there)
The thing about Slashdot is that even with some posters trying to be funny etc, you will come across many posters who know what they are writing about. They provide deep insights into stories that you would not normally get. One of the many reasons I try to read it every day - since 1997 when it began. The Digg crowd is not that sort - probably one of the reasons they bag Slashdot is that they know they will be seen for the bull**** in any comments/spam they try to bring to Slashdot. It is much easier to act knowledgable (ie Digg) than actually be knowledgable and express it (ie Slashdot)
Yeah, Slashdot is the real technical crowd. As Digg becomes more popular, it also attracts more immature users. A large number of these immature users are able to vote up more immature posts, which leads to a dumbing down of the site. Slashdot users tend to be elitists who will shout down people who make idiotic or immature comments.
Thanks for your replies, I subscribe for their Daily Headline Mailer and read them regularly, it seems they have employed a group of people or pay them for their comments though I am not sure as most of the posters seems to be same. I agree with davewashere, I too find they don't accept immature comments though they themselves make funny comments some time.