There are two basic kinds of people that play Blockland, we'll call them the noobs and the pros.
The noobs generally join servers with no real "gaming" value. They want a "casual" experience. These servers are generally sloppy and managed poorly. They are usually RP's, free builds, zombies or any server offering free admin. These people usually don't know much about the in's and out's of Blockland and have poor spelling. They think that a lot of people in a server means that they have fun little add-ons to mess around with or there are a lot of people RPing.
The pros generally join DM's, TDM's, jump/click challenge, and anything that tests their skills as a gamer. They are usually picky about what goes on in the server, how well it is managed, and expect the host and admins to know how to do more than just the basics and have proper spelling and grammar. They try to steer clear of a server they think might be run by a "noob". They think that a lot of people in a server means that it is run well and poses a challenge.
If you aren't known too well in the community for awesome builds or sweet add-ons, most pros will overlook you in the server list. Noobs will overlook it if your server doesn't look like it has instant fun. Both will overlook it if there are under 3 people in the server. And your server will remain that way because nobody wants to fight 1-on-1 in a server meant for there to be loads of people. So they don't join and the cycle repeats itself.
Now, lets say you want to to have an awesome TDM, so you download perfectly balanced weapons and other add-ons. But you want players do have more freedom than just runnin'-n-gunnin'. So you decide to let players build their own bases and manage their own teams. Now you have a basic "Fort Wars". Now all you need is people to join. You decide to name your server "(Your name here)'s Fort Wars!"
Lets say that there are two other Fort Wars that are also in the server list. We'll call them "haLoSONIcmArio777's fOrTwArZ!!1!" and "Thorax's Fort Wars". haLoSONIcmArio777's fort war isn't doing so well. The weapon selection is crappy, the builds are spammy, and random admins are banning people. Thorax's fort war has the maximum amount of players, but on top of that, players such as Space Guy, Ephi, and Badspot are all in it and having a good time, meaning more people are flocking to it. This leaves your server in quite the predicament.
Now lets pretend we are a player named "WeponGui". WeponGui really wants to play some Fort Wars today and noticed that there are 3 servers for him to choose from. He decides to check out "Thorax's Fort Wars" first because there are so many people playing and well known members seem to be enjoying it. WeponGui notices that the server is full but decides to try joining anyway, hoping that someone will leave. After a few minutes of trying to join, he decides that nobody will and moves on. He decides to try "haLoSONIcmArio777's fOrTwArZ!!1!" as it has a decent amount of people. After spawning he runs into numerous problems. People camping the spawns, overpowered weapons spawned randomly, and poorly made builds. He eventually gets mad and is about to leave, but is banned instead for the reason, "Y u soOoo m4D??" WeponGui then sees your server and sees the lack of players. At this point, he is frustrated he can't join a good Fort Wars and is not in the mood to be the only other person playing in a server.
Now, this happens with many people before you realize that noone wants to join your server. So you ask yourself, "How will I get them to stop by?" Well here is your answer.
Assuming that you aren't well known in the community, pros won't want to join your server right off the bat. Noobs see your server and think it will be too "challenging" and "unrewarding" for them, so they don't want to join. Now, your target is to get your server filled with pros, so how do you do that if they won't join right now? The answer is player count. You need a lot of players in your server to give it the illusion that it is very successful and well run, no matter how it is being run in reality.
Your first step it to get noobs to join. You already have your server set up for Fort Wars, so you don't want to abandon all that just to get a few noobs to join, so how to you persuade them? You need to advertise your server in a way that noobs will want to join. By changing the name from "(Your name here)'s Fort Wars!" to "(Your name here)'s Fort Wars! FREE ADMIN!!", noobs will flock to your server in an effort to become admin. You don't actually have to give it to them, you just want them there to add "value" to your server and to give it a try. Feel free to kick/ban noobs that spam chat and bricks, but be sure to give warnings to them first so you build a reputation of being "fair".
By now, you server should have decent players in it who are just building little bases and shooting each other. Try to get them happy so that they stay. Your goal is to keep players that want to play. If someone is killing someone else who is wanting to build and the builder is getting upset because they have to keep walking all the way back to their base just to put the roof on it, give the builder protection. Tell the person shooting the builder that the builder really would like to finish his build and you would like to impose a temporary ceasefire until he finishes. This should keep both players happy and your reputation should go up. If the builder keeps getting killed anyway, warn the shooter and threaten to kick him the next time. If it happens again, kick them. The builder will appreciate it and your respect will go up more. Also, the more respect you have from other players, the more likely it is that they'll come back to your server in the future, despite the amount of players you have.
After a while, you should have accumulated a fair amount of players who are playing by the rules, having fun, and posing a challenge to anyone who joins. This is when pros will start to join. Your main objective is to try to keep them happy so that they'll stay and invite their friends. But don't just abandon the other players in hopes that a few pros will play. There is nothing a pro likes more than to see a noob start to become a pro. Be willing to teach the noobs in your server how to play the game efficiently and give them tips. Be polite and try using correct spelling and grammar as much as possible. By now, your server is pretty much a success. Keep managing it in a fair, fun and helpful way and your servers will be a hit!
TL;DR - If you really want players in your server, you'll read it.