Maybe because in this post
https://forum.warmane.com/showthread.php?t=461207 the staff wrote:
"You will be able to experience the original spells, talents and mechanics, gear stats and functions of each expansion, without having to switch your standard WoW client that you own and use to play on Warmane realms with."
Sounds a bit like they promised a vanilla server to me, especially the part about original mechanics and functions of each expansion, but it turns out the QoL features are so impactful it can no longer be considered vanilla.
My point is, if you arent going to stay true to vanilla version of the game anyway, why not go full custom and give us arenas now.