If you think about it, it's absurd to expect them not to: Even if the community keeps its member list exactly intact forever, and the members remain at their exact same activity level forever, those very members will change over time. Hell, you will change over time and get to appreciate different qualities of that community.
Same reason my guild's forum feels completely different today than it did 5, 10 years ago.
An analogy would be if HN was suddenly overrun with rude, non-technical people attacking other posters and causing drama, and outnumbering the thoughtful and constructive comments and links we come to the place for.
The "eternal september" is particularly sad if you go back and read the archived posts of early usenet - I would kill to get discussion of that quality on the net today.