As the community grows, the definition of what's interesting to it gets more vague and distorted, like a drawing on a balloon that's continually inflated. And if it grows quickly, the graybeard:newb ratio drops. Both factors make it harder to enforce the interesting-ness norms.

I think this is a version of (or at least, related to) the concept of Eternal September.

http://en.wikipedia.org/wiki/Eternal_September