Feudalism has a lot of advantages in sf. It's familiar, but it's not what we're doing at the moment. Because it's familiar, the titles and such have emotional resonance. Most of us feel superior to it, but many of us are still kind of fond of it. And it's simpler than democracy, or at least can be presented as simpler, and it has fewer boring details.

Logically, societies with larger populations will be more complex, especially if they're dependent on complex infrastructure, but that's a lot of weight for fiction to carry.