They said they'd make it permanent if it gained enough traction and a consistent play rate, which it didn't to the extend they wanted.
However it got praised and liked by the playerbase enough to give it a second run after fleshing out the graphics, map layout and events which some were the reason behind players not liking the game (the alpha look of the map or some event rewards being just unfun).
If it gets popular enough this time it will become permanent after they apply a fresh coat of paint on the map and flesh out all the bugs.
Personally i hope it stays permanent after this time, the events are way more fun this time around (the snowball event and caytlin reward were just bad which got removed).
Edit: forgot to mention that if the gamemode doesn't become permanent it will be added as part of the rotating gamemodes.