I think they should have updated to 1.12 when it came out, however there are some good reasons why they may have chosen not to update since 1.9:
The 1.8 Network:
As Eggwars and Skywars (the two most popular games) are on featured on the server in 1.8, they purposely don't want to include 1.9+ features other than the PVP in either version. If the maps, chests or villager shops included 1.9+ blocks / items, then either the 1.8 games would have to stop being updated with the new stuff, or a lot of work would have to be done to find alternatives to all the 1.9+ update features for 1.8. They would effectively have to start managing an extra two games as they would soon become very different.
Shields:
In 1.11, shields were updated to block 100% of damage. This was received quite negatively by players, and for nearly all games that currently included shields a lot of balancing work would be needed to make them fair in-game. There are many solutions to this, like greatly reducing the durability of all shields and providing plenty of countermeasures like lava buckets, however the time it would take to make them work for all games whilst keeping the mechanics consistent throughout would be an arduous endeavour.
Lack of Useful Features:
Arguably, the updates since 1.9 were not as big as previous ones, and many new introductions were geared towards survival mode Minecraft as opposed to features a non-vanilla server could make much use of - this is especially noticeable with 1.11 being the Exploration Update. In 1.10, I'd say the only useful features for Cubecraft would have been 4 blocks, 3 mobs and 1 particle.
Development Time
Updating the servers and its many plugins to a new version, even without implementing any features, takes a lot of developer time that could be used elsewhere. The updates since 1.9 have come out more frequently than previous ones, so updating often would likely be costly and slow down other projects.
Accessibility
Simply put, the more versions that are supported, the more players that can join the network. Many smaller servers did not update to 1.10 or even add 1.10 client support for a while, so staying in 1.9 makes it much easier for those players forced to stay in 1.9 to connect.
I do find it quite hypocritical that we haven't updated since risking and committing so much with the 1.9 update; we effectively made being the most up to date large server our unique selling point, and now after our losses we won't capitalise on the much better received updates since.
