It is difficult to keep a server up-to-date when the way the game works is being constantly updated, and thus effecting the compatibility of plugins each time a major update occurs. Even if just 1 plugin doesn't work, it can cause major problems which can even cause the server to become unstable and crash often.
The supplier of all the plugins does not recommend updating servers unless they recommend it, and they have not yet recommended that any server updates past 1.8.1 yet.
Personally, I'd prefer an out-of-date server that runs smoothly, rather than an up-to-date experimental server that is down all the time and can run limited plugins.