According to the developers, they are citing that this is due to the quality of the mode which they feel doesn’t quite live up to their expectations. In an update posted onto Reddit, the developers write, “Today, after extensive evaluation we are sharing that ZOMBIES Mode is being removed on March 25th. We had always mentioned it was limited but with no explicit end-date. We wanted to see the reception, see the feedback, and see how we can potentially shape the mode for the future. However, the mode just didn’t reach the level of quality that we desire.”
However, it should be noted that this doesn’t mean that the mode will be gone for good. The developers aren’t closing the door on bringing it back in the future, but only if they are able to ensure that it meets their standards. “We may bring the mode back, with Nacht Der Untoten, once we can make sure it is high enough quality, but for now we’ll focus on development for Multiplayer, Battle Royale, and Ranked Mode.”
When exactly that is remains to be seen, so if you do enjoy the Zombies mode in Call of Duty Mobile, you’ve got a couple of weeks left to enjoy it, so you’ll probably want to take advantage of that before it is removed from the game.