I think there are two key reasons:

1. KiwiFarms was explicitly created for essentially that purpose

2. It's far more extreme. Part of the reason it exists is because people on these larger sites (notably, 2000s-era SomethingAwful) were getting progressively more concerned and uncomfortable with the behavior. Sure, it's a concern in other places, but there are checks there. KiwiFarms has none of those checks because the behavior is the intended use of the site.

So, sure, you could go after some of these other places, but KF sounds like a very good starting point.

Mobs always attack the weakest link. KF is not widely known- if it were canceled basically nobody would care.