Several things may cause people leave.
1. There is nothing more to unlock after you unlock the weapons and the 4 helmets.
2. They just don’t like the game.
3. The game has a lot of stuff that needs to be fixed and they are not patient enough.
4. The patch/update/hotfix takes too long to be released.
5. They want to kill people without effort.
6. The competitive aspect is not famous enough for big tournaments. (Compared to other games)
7. They don’t want to spend the amount of hours it takes to learn how to play the game properly.
8. The game is not very well optimized.