it's because when people get bored, they move, and when people move, others follow them. people have a tendency to follow a sort of pack leader, and when one person says "this sucks, i'm leaving" there are usually going to be some people that follow them. that creates a chain reaction that sucks more and more people away from your server. what i would recommend doing is resetting your server map after a set amount of time, creating a some type of economy(not mod based), and having mini events(such as spawning a bunch of mobs on a town, having people fight an enderdragon on a mountain, ect.). that would keep people from getting bored and leaving. unfortunately there would still be people getting frustrated because of things such as griefers, lost items, and major forgetups on their part. while the server was very popular, you forgeted up with the war server. many people decided "the map is being reset tomorrow, not gonna play. war server? not my style" and migrated to a new server, leaving yours in the dust. the only way i can think to revive your server is to advertise a complete reset with a spawn in a good location(not a stuffty jungle or something stupid like that, probably in some kind of player built town that you build over time). you also need to update the server as soon as you possibly can, updating before other servers will give you a massive edge over the competition. one last thing, you need to make things dynamic and centralized. a problem i noticed in the old server was places got abandoned quickly, causing a fairy ring effect. as the building potential at the spawn diminished people moved farther out, decentralizing the server and making it seem unapproachable and very empty to anyone who was brand new. that's a problem on any server that has an ongoing map, either you update it or you have to keep the spawn area useful and appealing so people actually go there a lot to give your server a lively feel, as opposed to the lonely, sad feel of the old spawn.
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