Hey man, I can understand your predicament. I have 700ish posts, mediocre karma, and no feedback and I probably won't get invited either.
However, this is the first year something like this is being done so it is an experiment as well. Saying you have to have x number of posts or y amount of karma to get to join is a very generalized way to go about it but when this many people want to participate it is hard to go on a case by case basis. It isn't a perfect system, and hopefully next year it will be tweaked to work even batter.