For one, they are locked behind a reputation and a $25 race change (or a 200 hour long level grind if you roll a new toon). Second, Humans, Night Elves, and Blood Elves are far and away the most popular player races in the game, and make up a combined more-or-less 43% of the game's population. Of the remaining more-or-less 60% of players, Kul'tians have to compete with 18 other playable races. Doesn't help that the female model looks very unappealing. If you want a rugged looking female warrior, the other races do a better job, so one's investment in Kul'tirans depends on whether or not they prefer the male KT over all of the other options available. Statistically speaking, you're just not going to see many Kul'trains.