Because all social platforms and message boards are like living entities:

They have their infancy and grow into something popular (relatively speaking) then your older members either get bored or jaded, often both. The culture of the platform starts to change while more people leave disliking the change than new people discover the platform. Resulting in the platform steadily shrinking in popularity.

Every social network and forum has experienced the same phenomenon.

It's hard to describe the experience now, but Slashdot in the early 2000's was more of a community disguised as a tech blog. Sure the editors selected the stories from the queue, but everyone had a chance to submit something they thought was interesting, and then comment on what got through. Even the site editors would regularly participate in the discussions, and I got the feeling that everyone there (except maybe the trolls) were passionate about technology. It was easy to spend an entire day just going back and forth with someone in the comment sections about whatever the controversy of the day was. It was magical.

Once CmdrTaco sold the site to Dice, they tried to turn it into a business intelligence/job board, which turned off a lot of long-time users (they also tried a site redesign that was functionally useless and had to abandon it after a lot of complaining). Then Dice got tired of it and BizX bought it to add to their trophy case, but haven't done anything meaningful with it. The site has been on autopilot since the acquisition. The 'editors' are faceless interchangeable usernames who just load up a bunch of stories from the queue and let them auto-post throughout the day and almost never show up in the comment threads. The 'community' is barely there, most discussions get less than 50 comments. They turned off anonymous coward posting (unless you're logged in). The user poll hasn't been updated since April. And so on. It also didn't help that Hacker News came along and ate most of their lunch.

Basically, slashdot was hollowed out and the shell is all that's left. But the new owner can put 'COO of Slashdot Media' on his LinkedIn page, which is probably the most important thing.

IMHO people moved to Hacker News. I would guess one of the reasons is better comment system here. Algorithmic ordering of comments based on up/down is important for scaling to thousands of comments. Slashdot had scoring system, but comments were still ordered by time, which makes it less practical.

Stories were selected by site mods (editors) and that's censorship, man!

It was sold and the new owner didn't like the way it was.