One of the reasons that Krath wasn't used in kotor 2 is that Krath means something nasty in French. I don't know but Sith are more famous than their lesser known cousins.
Personally as Enemies, Krath needs more screentime.
Edit:Chris Avellone has claimed that the Krath did not appear in Star Wars: Knights of the Old Republic II: The Sith Lords because the French translation of "Krath" seems to be "something pretty bad". In fact, it sounds like the word "crasse", which means organic dirt