I play Orion - they at least have some level of handsomeness in them. I didnt want to play Klingon simply because they are ugly brown mutants. If they looked better maybe people will start playing them.
The Klingon Faction is pretty much full PvP right now and most PvP players will sacrifice anything to improve their chance to win, even by a very small amount.
Klingons get to chose two traits in addition to their two ground-based racial traits, Aliens get to chose all four traits = Aliens make "better" space captains and have the potential to make better ground captains too.
Then you also have the fact that most races on the Klingon side offer extremely poor customization (Gorns, Letheans) or don't even look like they did in the series (Nausicaans). Hell, even the faction's "main race" has far less customization options than the majority of those on the Fed side of the fence.
The "Ugly Factor" plays its part too since "Ugly" races are usually less popular in most MMOs, especially when you have the option to make a "more beautiful", fully customizable and more efficient character of the same faction without having to suffer any penalty
So yeah, the only "real" Klingons you'll see in PvP are the people who really liked the race and wanted to make one no matter what (well except for those who rolled an Alien and made him look exactly like a Klingon to get the best of both worlds), I know that's why I made mine.