Well there are a couple of reasons why the servers haven't been updated and WowJudo has already stated theses reasons for why the servers won't be updating for the time being.

(Note: down below are Judos words not mine)

1. There's like 3 versions of pixelmon there now, give it a month and its 5 versions. It isn't really optimal to expand the server with so many different mods as their maintainance and setup become harder
2. The makers of the mod are unknown to me and the quality of their code is unknown too. Which creates the potential issue of bug patching issues
3. Lastly the makers of 5.1.3 or other variations of pixelmon mod might include malicious code that destroys the server or creates a backdoor for them
4. If we would decide to update to new version.. what do we pick? Making 3-5 more servers with different version and equal ammount of modpacks is not really an optimal solution