I think Slashdot died when Cmdrtaco sold out, not for any real nefarious reason, but simply that it was no longer "a list of things Cmdrtaco was interested in" but had spread so wide that it was more of a mish-mash. Early on it was nice, because if you liked the things Cmdrtaco was interested in, you'd like almost every post (even if you got mad at it). Once it was wide, it was harder to follow.

Digg had a similar issue, but was more "active" for awhile. Reddit really "solved" the problem by having subreddits so you could curate what you were interested in.

Also, the people who were using /. grew up, and there wasn't really replacement people.