Why Have These Types NEVER Been in the Elite Four?

Despite having 18 types in Pokémon, and plenty of Elite League Members over the years, we have yet to see every type represented as one of the top Four Trainers in a given region. So let's figure out why these few types have been snubbed for so long and maybe try to pinpoint where they might have been best used across the various Pokémon Leagues, (and for the purposes of this list I was not counting Champions, not that it would change the results anyway...)