The good OT servers die off because of many reasons. I owned a server (Frozenhell), and looking back, many things led to its downfall. The most notorious reason however, was corruption. Lets go slightly off-topic..[skip the green if you just want the point]
Ultimately, my server closed down because near the end, my Paypal looked like I had just purchased a car. It had gone negative a couple grand (from +$700~) from a magnitude of consecutive and planned chargebacks from numerous people. My Google Checkout account met the same fate, and I was not able to win any of these cases because of how it happened. Paypal and Google would simply look at my Case History, and automatically assume that I was a scam and that these people that successfully nuked my account were my victims. I'm sure there probably aren't many with my story, but that is the reason there is one less decent server.
That was only the tip of the iceberg, however. It had began its decline months earlier, when one of my so-called friends and staff members, Jason--whom I met online, decided to release the data directory. Apparently I wasn't giving him enough of the profits. That server was never made for profit, and as such, didn't profit quite as much as he thought. There were donor items and private spawns, but most people traded internally once they owned them. You could say this was my own fault for allowing a virtual friend that much access, but I lacked the time to accomplish the tasks that needed to be done by myself. "Great fucking idea, lets support the lazy greedy worthless server owners who just want to put up a Frozenhell clone and fatten their wallets."
At that point, I gave up as Administrator and handed the reigns to my second-in-command, Lessaire. As far as I know he didn't do any direct damage to the server, except for the fact that people tended to dislike him. I know, because they told me, and threatened me to quit if I didn't oust him, which I didn't. Tough luck, right? Lessaires goal and only interest was (and probably still is..) maximum efficiency. He would spend countless hours revising every line of code that server had, as well as maintaining and tuning the performance of the dedicated. Amazingly useful to me, no use to the community. They did not care about his behind the scenes work. They just wanted a flashy frontman and weekly updates.
The server slowly went downhill from there. I wasn't around to ensure my projects safety, and because of that, it died. You can't trust anyone but yourself over the internet. No matter how pure of heart you think someone is, they are not going to have your best interests at heart 100% of the time. Sooner of later they will mess up. Greed is a worldwide plague.
The moral of the story is this: In my opinion, people kill the good servers. Whether its corruption on the staff or a bunch of cheating douche bags. One thing is for sure. The key to a good custom OT, is a good staff and a good community. They go hand in hand, you can't have one without the other. Unless you play on a real-map server, where staff interaction can sometimes be non-existant, which is quite possibly why they are more successful.
Let this be a lesson to any future server owners. First, if you plan to succeed, minimize your need for outsourcing your staff. Second, if you plan to make money from your server, don't be a lazy ass. Invest your time, because time equals money. In the beginning of my server I knew this. When I became ignorant and forgot, my server went spiraling down. Finally, a server created for the sole reason of earning a paycheck will never last. Then it becomes a job, responsibility, and a duty. If your eyes aren't constantly blinded by dollar signs, you will get much further. The money is just an added reward.