Sufficient Velocity is not a sci-fi site either.
It's basically Spacebattles 2.0 (or rather 1.1). Sufficient Velocity is the splinter site that separated from the Spacebattles over the mod disagreement, but their community is compromised of the same people. If someone has an account on Spacebattles, he or she have the same account on Sufficient Velocity, the only exception is when you are banned from one but not from other. You could imagine SB and SV as identical twins, indistinguishable from each other in every single detail, which act the same, look the same, and even think the same, but then their parents have an argument and one dyed her hair blue...
Neither SV and SB are sci-fi sites per se, but you certainly could post there.
Their audience doesn't look for sci-fi or fantasy, they look to satisfy their inner munchkin urge to min-max everything, and rule lawyer out of everything. As a result, they love fan-fiction, not necessarily sci-fi but the setting where there are established rules, and then they want to break that rules because they are smarter than the universe.
Sufficient Velocity is the reference to their obsession with calculations. As everyone knows, there is no such thing as "enough dakka" and thus there is no "sufficient velocity" either, hence the name.
The original fiction isn't popular as a result. Not because of the genre, but because no one knows what the rules are. If no one knows what the rules are, where is the fun with outsmarting them?
That's why it is mostly Worm.
Worm is the SB original superhero setting (hence theoretically sci-fi) where powers work the specific way, and couldn't work otherwise, hence every single fan-fic ever created revolves around outsmarting the universe. Since everyone knows what the rules are (everyone on SB and SV read Worm), they could rule lawyer around then. It has very little to do with science, and everything to do with the established mechanic you bypass because you min-max, or not obey them at all,
You won't have much success on either if the audience couldn't tell what is the trick is.