I assume they didn't file lawsuits before because, unlike NovaRO, most private servers weren't actually a registered company (LLC) you can sue.
But I'm not an expert.
And it's really hard to tell with a sample size of "1".
The only thing having an LLC does is make it easier to sue people related to the server. If there is no business entity it can be harder to sue related people because you need to file suits in their jurisdiction. With an LLC in play you can sue everyone from one location.
Questions I'm seeing asked often on Reddit and other places are, "Why NovaRO?" and, "Why didn't they go after rAthena?" My response to this is, "What if they are?" The emulators on their own would be difficult to sue because they don't really use any of Gravity's IP and due to the crowd sourced development it would be difficult to rope everyone into a single jurisdiction. Gravity would likely have to sue each dev individually.
However, as I've explained, because of the LLC, anyone associated with anything they did can be pulled into the local jurisdiction/court that Gravity is suing in. Gravity has 10 yet unnamed defendants in the case. NovaRO has fewer than 10 devs and more than 10 staff, so those 10 are unlikely the staff of NovaRO. Nova is listed as an rAthena dev, he goes by the same name ("Nova"), he has NovaRO as his profile picture on rAthena, and that makes it pretty easy to prove that the rAthena staff knew he was running NovaRO.
Imagine your friend asks you if they can borrow your car, you say yes. Your friend then uses your car to rob a bank. You didn't know that was going to happen and lending your car to someone isn't usually illegal, so there is no issue here. However, if your friend said, "I need money so I'm going to rob this bank, lend me your car." and you did it, well, now you've become an accessory the robbery. Even though the act of lending your car out isn't normally illegal, you did it knowing an illegal act would be committed with it.
Using this same logic, it is possible that Gravity intends to pull the rAthena devs into the NovaRO case, arguing that they are accessories that knowingly aided Nova in committing the crimes they allege. While it may be possible for the devs to fight this, they would have to hire lawyers to do so, and even possibly travel to California themselves. For many, this may not be financially possible and they will lose the case by default.
Now, it's entirely possible this is incorrect, no one knows Gravity's intentions right now but Gravity. However, it is possible this is their plan and why they are suing this particular server at this particular time. While other servers have had devs on both the server and emulator project this is one of the first very popular servers where the owner of the server was a dev. Prior to this, it was typically a dev in a lower position within the server.