I thought GalactiCraft wasn't updated for 1.12 yet.
Decided to look at the their forums and it doesn't have a 1.12 version only 1.11.2.
Here is the link so you can understand what I'm seeing.
EDIT: Looking at the other options, all three of them have there own problems. I'll go over each of there problems.
Factions has been on the server before and it eventually became how Prison/Survival+ are atm, bringing it back would only show why players don't much care for it.
Galacticraft/Tekkit/Modded I know I'm grouping a few things here but they're all in the same category "mods". I feel only a select few players will ever play on a modded server made by DS, since the average player has the computer of a dying toaster. Diamondshaft has actually had a modded server as well, but it was eventually shut down since only 7 players played on it, the majority of those players were staff members.
In conclusion, I feel a lot of the issues on why a server tends to lose activeness and never increase in player base is because DS doesn't have a hub. Adding a hub could draw players into those other servers but could also drive them away from survival, and tbh its really up to Anthrax on whether or not he wants to add a hub allowing more players to see the other servers that are on Diamondshaft. At the moment there is a message that scrolls through chat stating about DS having other servers, but players most likely ignore chat majority of the time, which is why you don't see more activeness on those other servers.