sDirt Call Of Duty BlackOps 4 has only three phases, unlike the rest of the parts, and they are: 
1- Zombie mode
2- The cooperative phase
3- The Battle Royale mode
The story phase was not released with the game and they replaced it with the battle royale phase, and the reason for that is that they need a lot of money to develop in the story phase, and also because the number of people who play the story phase compared to the rest of the phases. That was the first risk Treyarch took, now let's talk about their other risk in zombie mode.
1- Ease
What distinguishes the zombie phase is not only the fun but the challenge as well, and the challenge is what makes each player challenge other players to reach higher rounds. The challenge was based on many factors that I will mention below, and each of these factors strongly influenced the zombie phase, causing it to decline and players abandon it quickly, which led to  The lack of challenge in the game for several reasons:
1-  The game starts with a jack-o'-lantern
In all phases of zombies in the previous parts, the first factor in the difficulty was the death of the character by two or three strikes, and in order to increase the level of health, you need to buy the jacker perk, but with Black Ops 4, death became from 5 consecutive strikes without even purchasing the jacker perk. In this part, they added the feature of choosing the beers that you want, and the limit is only four beers, and for that reason they removed the berserker berk because they would know that all people would choose it and there would be no diversity in the beers, so they decided to remove it and make it automatic with the character, which caused a decrease in the level of zombies.  

2-  The game starts with super power
The supernatural ability was added in the Black Ops 3 part, but in order to obtain it, you must solve a puzzle or a challenge, but in Black Ops 4 you can start with that ability and also choose any ability you want. And that was the problem, you start with all the things you need to survive in the first round. And you can also charge the ability faster, unlike the previous part, which made it a lot easier, but it attracted a lot of new players, and in return they lost the old players who loved the difficulty of zombies. 
3-  The game starts with powerful elixirs (GobbleGums)
Elixirs are used just like gummies in Black Ops 3, but only with a different name, and most of them are copied and pasted from the previous part, but the problem with this part is that you start with powerful elixirs as well, such as moving to the fallen player instantly and moving to any random place on the map. These elixirs made zombies much easier than the previous parts, which were distinguished by their difficulty and challenges. This is one of the reasons why many players give upZombie mode quickly.
3- Difficulty unlocking weapon colors
After they decided to make the zombie phase much easier than the previous parts in order to attract new players, they also decided to make the requirements for unlocking colors difficult. Really weird! In order to open the colors, you need to kill zombies with headshots only, and the problem with this requirement is that it is impossible to kill them with the head in the high rounds, and there are also many fast zombies that you want to kill quickly and they explode during their death. You have to kill about 3600 zombies with the head just to unlock most of the colors! If only they had made killing them the only requirement to unlock the colors it would have caused the players to play it for a longer time but unfortunately they decided to make it easy in all respects but they made it hard to unlock the weapon colors.