The majority of Castavarns are found in the Kay'Le'Eeska'Rarns. But if that's so, then who's to say, why Kasaviirs are not their prey?
You, of course, along with what Castavarns and Kasaviirs are, and what sort of place the Kay'Le'Eeska'Rarns are.
Note: "Kasaviirs don't live there, stupid!" or some other thing that defies standard expectations is, of course, quite welcome