I think there might be a simpler explanation even than that - I suspect HN took over from Slashdot simply because it has a rich corporate backer, thus doesn't really have to make ends meet using banner ads. Slashdot did for a while too (VA Linux) but eventually the gravy train ended. Banner ads really make the experience worse, and they don't work well on sites targeted at geeks due to rampant ad blocking. Lacking any form of banner ads probably also contributed to allowing the HN UI to be extremely dense, so the throughput of topics can be higher. Technical people like density, fast loading, and relative lack of ads.

The heydey of Slashdot was when it was itself in its 'startup phase' with Rob Malda leading the charge from the front, and back then it was great. Complex and sophisticated technical discussion, as well as other topics. Arguably way less politics than HN, though maybe more open source-oriented politics. Not many ads. Useful-ish summaries.

Frankly the question why HN is more "successful" than Slashdot rankles somewhat because I feel like it's maybe not really more successful, depending on what you define the goals of such a site to be. To me it's just different. For example, Slashdot threads did a great job of mixing discussion and entertainment. They made me laugh, they made me cry, they taught me a hell of a lot about UNIX and programming and often they did all these things simultaneously. Some people don't like entertaining comments but Slashcode gave people great UI controls so you could hide them if you didn't like them. In practice, I found they made discussions way more enjoyable to read. In contrast, HN basically just bans entertaining comments. It's a crude approach.

Slashdot also pioneered a lot of ideas and tech that made very big discussions work. Back then sites got "slashdotted" because Slashdot scaled far better than many of the sites being discussed. Contrast this with HN, which can't really handle threads of more than 500 posts before the UI breaks and around 2000 posts, the site itself starts to break.

Finally, frankly, Slashdot moderation was way less frustrating than HN's can be. HN claims to a high minded search for intellectual curiousity, or some such. DanG makes it work, more or less. But absolutely nothing is formalized in any way. How would you replace Dan? The principles which are claimed to be enforced are sufficiently vague and inconsistent there's no way you could ever scale the model up. Slashdot, for all its faults, provided a crisp and actually formalized moderation system that tried to reflect the priorities of the user base rather than Malda himself.

The current site is a shadow of its former self due to neglect and being out-competed by other sites that were able to keep on the VC train longer, but the core ideas of the site worked. Honestly I miss it sometimes.