I'm not sure if the analogy of a blog works. The problem with blogs is that they are disconnected from the online spaces that people actually spend time in. If somebody were to create a really great subreddit and make it flourish, it would appear on r/all or people would link to it in other subreddits, people would hear about it, it would grow. A blog could be filled with great content and be entirely ignored because nobody would ever come across that blog in the first place unless they happened to google some very specific terms and wade through dozens of other search results. Even then, unless they actively remembered to do so, they likely wouldn't keep up with new blog posts because there would be nothing to notify them.

Saidit and sites like it benefit from the fact that they can be made up of countless separate communities whose content can be funneled into a stream of content that's individualized for each user depending on which communities they subscribe to. This individualization means that users and entire communities aren't required to keep up a high level of quality for that stream of content to remain high in quality because they are so easily ignored by unsubscribing or never subscribing in the first place. Communities that users consider interesting, high in quality or truthful will stay in their content-streams and have higher engagement while others will either remain small or wither away. People are allowed to post whichever content they like and build communities the way they want and users in turn are allowed to focus their attention on whichever communities they deem worthy of it.