I think it's that most people don't know how to make combat arms private servers, they're probably much more complex than making a Runescape server or a Maplestory server. And, private servers are against the TOS for pretty much all large online games, it's just that Nexon and Jagex can't get rid of all of the private servers, just the popular ones. And since virtually no one knows how to make a CAPS, it's easy for Nexon to get rid of them.
Just my theory, don't hate :P