Because groupthink is inherent in the upvoting/downvoting system. Perhaps only Slashdot is the only community I've seen where points are awarded to comments not based on agreement, but on general merit. Admit it: you've downvoted people you don't agree with. When enough people do it, you get groupthink.
Forums/mailing lists don't have that problem. Every post is given equal merit in the number of eyeballs that read it. Those comments that are not interesting can get ignored, those that are interesting will either merit a "yeah, I agree" response, or a "you're wrong for [x,y,z]". That is a dialogue. Community moderated upvoted/downvoted discussions are not a dialogue. They are a "here's what we all think" metric.
Because groupthink is inherent in the upvoting/downvoting system.
I'd almost argue that groupthink is inherent in human interaction in groups larger than one. Almost.
Forums and mailing lists are certainly susceptible to it, even though it occurs less often, but I suspect that has more to do with the size of their memberships. SA, for instance, has tons of it.
Oh, and /. had a serious problem with it for a long time. Their moderation system helps, and maybe it's gotten a lot better (it's been a while since I was active on it), but oh man when /. was the site, it was pretty bad.
/. is a good example of what seems to happen to a lot of online communities -- a small, competent, awesome base of users is very active for a while, things snowball, you then get a huge upsurge in users and activity and trolling, and then that eventually fades and your userbase stabilizes at a point where the userbase is pretty solid and there's not too much in the way of ridiculousness going on.
Forums/mailing lists don't have that problem. Every post is given equal merit in the number of eyeballs that read it. Those comments that are not interesting can get ignored, those that are interesting will either merit a "yeah, I agree" response, or a "you're wrong for [x,y,z]". That is a dialogue. Community moderated upvoted/downvoted discussions are not a dialogue. They are a "here's what we all think" metric.