My guess is because there's very little demand for them? There are a lot of fish that are really common in South America, but they're not exported or fished for because there is no demand. Chaetobranchus come to mind. I read somewhere that you can find them easily but Rapps only has them once in a while. The same goes with Heroina, which I've searched literally every corner of the internet for. You see notatus and rotkeil for a reason.