It is because the Master Server still isn’t quite in shape… yet.
Unfortunately, the Master Server drops any listed server after a few minutes. This is being looked into and will hopefully be solved in the near future. In the meantime, the server log still lists the most recently created servers, and ones closer to the top can still be up, even though they do not appear on the main list. If you write down or remember the IP address, you may be able to join by IP to these servers.
A link to the Master Server list can be found in the left hand side of this page, under the header “Community.”
We apoligize for any inconvenience.