I think a large part of the reason hackernews is good is that operator's incentives are more closely aligned to the desires of users than average. Ycombinator benefits if HN is the best place to discuss the creation of technology/software because it boosts their brand and gives their companies a communication and recruiting advantage; most other platforms are primarily interested in ad revenue and are only incentivized to provide a good experience to users as a factor in that equation.

Defense against spammers/bots is a tough problem though. Having great moderators and users who are savvy and intolerant of bad content probably helps but I think it would only go so far on a large site. HN might benefit from the relatively narrow appeal of its content in that regard.