Hey there~
There's specific reasons for some servers to be in earlier versions. Parkour, for example, is best played in 1.8.9 due to the changes in hitboxes and the way blocks work are very different compared to the most commonly used versions currently (1.12 and 1.15). If you play parkour in later versions, you will have a very glitchy experience and many of our older maps won't work properly.
I'm not entirely sure on the specific reason for servers such as Factions and KitPvP to be in 1.8.9, but I would assume it is related to updated combat in later versions that is more annoying (and disliked by most PvPers). Thus these PvP intensive game modes remain in earlier versions.