Races are a strange beast.
WotC seems to be treating them like classes, which isn't the smartest idea. Classes are more modular: introduce a new class, and your character can then multiclass (assuming there's multiclass feats?) pretty quickly, gaining its powers.
Introduce a new race, and you are saying: "Time for a new character!" You can't retrain yourself as a killoren, or as a gnome. It's one of the few immutable parts of your character sheet in 4e.
So they're less easily usable; you need fewer races than classes in the game.
What you do sort of need is a way to turn any creature into a PC race. Some people like the standard fantasy milieu, some people wanna play beholders and sentient squid. You will never be able to anticipate the races people actually want vs. the ones who will never be used. The most efficient solution is to make creatures easily convertible between the DM's side of the screen and the player's. 3e tried to do that, but didn't do so great of a job. 4e has abandoned that idea, and so needs to play the odds a bit more. As they run out of races with significant game history, they're going to wander more and more into the realm of "why should I care?"
A new race only adds to an ongoing game a cast of NPC's and a potential race for new/dead PC's or re-started campaigns. In 4e, the weight is more on the latter, since NPC's don't need stats, and player stats can't be used as combat stats. Each new race says "Start a new game with me!" Of course, given the duration of most D&D campaigns (and the reasons they fall apart), and the rarity of true death (especially in 4e), they aren't even very useful for that. I suppose it allows characters to "retire?"
People are saying that the killoren sounds more like a monster but, really, almost any new race is going to be pretty monstrous. Heck, the dragonborn, and the eladrin are pretty monstrous, as-written.
A lot of the time, when a race is seen as a "monster," the DM usually means that they aren't mundane enough -- a lot of DMs in this thread don't want killoren wandering around their cities, or farming the fields of their town. It's important, usually, to have a PC race capable of doing mundane things: it makes the adventurers of the race more exceptional. Human adventurers are awesome because most humans are dirt-farmers. Killoren adventurers might not be so awesome, compared to other killoren.
4e, I think, has difficulty making anything mundane (similar to 3e before it). It's intentional, but it also works against the goal of providing new PC races -- a goal which, as I pointed out above, has limited returns in the first place.
I like weird races, and the race itself certainly has a place in my world, but I'm not holding my breath for it ever to be actually used, unless maybe I get a chance to play.
