Guess:
1. as the number of servers increases, the number of bad servers and good servers increase at a steady ratio (9:1?)
2. as the number of bad servers increase, the average good user feels more overwhelmed by the sight of so many bad servers (could be relieved by some fancy server browser updates maybe?)
3. as the user feels more and more overwhelmed as the server count increases and increases, he / she is more likely to not join a server at all.
4. The "bad" users join the bad servers. Most will sort the browser by playercount, which means only bad servers will appear on top and this becomes more polarized as the number of servers increases. Any "good" users continue to not join servers, which causes more good servers to either stay at low playercounts or cease to exist.
5. As such, a steady increase of bad : good servers at a set ratio will result in "bad" user over-activity and "good" user underactivity. The "bad" user's standards are lower and are prone to join high playercount servers.
TL;DR: if your server has players you're doing it wrong.