To answer this for you, the Cathar were very nearly wiped out during the Mandalorian Wars, or more pointedly The Battle of Cathar... I'll do a cut and paste from the wiki for you rather than say the same thing in different words but here you go..
So for what ever reason they don't make an appearence in the movies .. though I believe there may have been a Cathar Jedi in the the large fight scene of Clones.. I can't be sure.. but they are very much there in the lore of Star Wars.