I think you can generalize that to just one reason: the crowd became too large. It grew to the point that most articles were about things that any given reader

I think you can generalize that to just one reason: the crowd became too large. It grew to the point that most articles were about things that any given reader wasn't deeply informed on. So the deep technical discussions produced far less chatter than the more general-interest/less-technical discussions and faded out of view. You also saw a bevy of those less-technical people offering their less-technical opinions on technical articles, whenever those things contained some keyword that registered. And, because there were so many more of them, this caused thread quality to suffer. So politics floated to the top, technical discussions became mired in less-technical opinion and technical folks left. Reddit isn't much different. It just attempts to duck the growth problem by enabling splintering into sub-reddits, which in turn splinter, on and on. And instead of "leaving reddit" the way people just stopped reading /., people just leave various sub-reddits. Which, as far as discussion goes, is effectively the same thing. The old forums becomes noisy, generalized, politicized and hollows out. People leave to find new ones. People may not be leaving the reddit platform but that's a distinction that really only matters to the people running reddit. (and maybe someone who's really attached to their profile/karma/whatever)