It's isn't fair not to update a game mode and say players dont like it. If MP and Br were neglected and zombies was getting all the attention of course people would prefer playing more of it.
If you want a fair count pls take it from when undead siege was added in 2021-22.
And zombies has been a in Cod from 2007. Its not like there is a shortage of zombies content to add, if you start adding a new map from WaW to BOCW there is literally a ocean of content.
And many assets of zombies like bus station, diner , farm and Alcatraz have made their way into BR mode with a little effort they can make Tranzit and Mob the Dead too.
I know money is a part of the reason but I'm sure there is a fair way to monitize zombies.