This idea does have some promise, but there are several areas where this is more difficult. I am only experienced with Parkour and Creative, but here are the reasons why those servers probably will not update for a while:
Parkour is very buggy on any version after 1.8.9. Heads glitch, panes glitch, etc. Because of these bugs in the game itself, parkour is unable to update.
For creative, there is seemingly no reason why we shouldn't be updating. However, it is important to realize that updating an entire server takes a lot of time on the developers' part. They are real people with real lives and updating the server, while it has been suggested, will take a while, because they just have so much to do and so much we can't see.
I'm sure other servers have reasons why they aren't updating, but these are just the ones I'm most familiar with. I hope this offered some clarity.