Exactly. Nothing unique about your server. Apart from a pretty much raw RL map there's nothing that will attract players. To achieve that number of 500+ online you have to work hard and dedicate time.
All those top RL map servers apart from the basic RL Tibia stuff got also a lot of different, unique content. You can't be successful if you blend with the crowd, you gotta somehow stand out from the rest.
Next; advertising. Apart from having a well built server with lots of interesting content, you need good advertising. If you want to see yourself on the first half of otservlist then you'll have to sink quite a bit of cash as afaik it's not that cheap. Sure, you can make it out even without shelling out lots of cash, but in that case, it's gotta be a damn good worth playing server.
Having a reputation behind can also help quit a bit but not neccessarily, though this is just my opinion. Most of the top servers have been online for long and/or have a well known owner which also contributes as players might feel more confident knowing what to expect. When you come accross a random unknown server you don't know whether the server will close in a week, is the staff corrupt or is it just a cash grab.
Well, neither you have a server that sparks any curiosity nor any kind of advertising, apart from this thread. Considering this, your server falls into this second group of OTS; one of hundreds look-alike not-special RL map servers out there with minor differences between each and all together don't have more online that a single top RL map on the list.
Sorry but that's not how it works anymore. You have to invest way more into it than you thought and it's one of the reasons why I quit working on my own server. It takes a lot of time.