It's the requirements, member is easy to get.. I use to ask players to make a comment on planetmc server page and give a diamond as it helps boost the servers reputation.. since I stopped asking for that before giving member rank, no one bothers. Go figure... why I'm reluctant to always just keep giving out stuff.
Think not of what the server can do for you, instead what you can do for the server.
Member rank is now tied to just 10 server votes, and its more automated rank promotion.. which isn't that hard to get, so no member rank itself isn't really a trusted rank, it just opens up slightly more for players.
And there are ranks that are easier to get than Member+
They are...
Forester -(albeit a gimmick rank)
Voter -( Being one of the top voters in the month, probably one of the easiest ways of showing you want to help the server and getting permanent fly)
Architect -(one most should be working towards as the server is about building cool structures and towns)
Artist -(a rank that I often think I shouldn't have bothered adding, as you're better off going with trying to make some nice looking buildings than pixel art)
If the ranks were just about colorful titles, then sure I could add a bunch more .... if the ranks about gaining more perms, then better come up with some requirements that match what you put in to help.
Last thing is that server has a monthly bill, and I pay it with the help of donators. If I just gave alot more plugin access and permissions out to every player who joined my server. How long do you think it would last, I can tell you I would get sick of running the server very shortly.
Trying to find a balance for all the stuff that keeps players who play for free having fun and not restricted in what they can do, like not being able to fly at all. Takes time, and its often more about what plugins are available to help me achieve that balance.