The in-game server list is broken. It actually pulls up only a very small percentage of the servers that are online, so it's luck of the draw as to whether yours shows up. Sometimes it does, sometimes it doesn't, sometimes you just have to wait an hour -- or a day -- and sometimes it never will.
Most people find servers to play on through a server listing site such as arkservers.net (not affiliated with us!) Your server will show up on sites like that.