This year’s Call of Duty title did not launch with a campaign, despite the fact that previous Call of Duty titles did. However it seems that at some point in time, Treyarch did explore the idea of introducing a campaign to the game, and apparently it was even in development until mid 2017 where they decided to ditch it entirely.
This is according to a post on Reddit which quotes a fire Treyarch QA tester who revealed the details. According to the post, a campaign for Black Ops 4 was in development which was designed to be a 2v2 co-op race to finish the mission. However apparently due to Treyarch wanting to make too many maps, the entire development was scrapped due to the lack of time. Instead Treyarch opted to work on the Blackout battle royale feature and Zombies.
Of course there is no way to actually confirm whether or not such a feature was in development to begin with, so take it with a grain of salt. However given that players and reviews of the game and its Blackout mode seem to be quite well-received, we suppose it was not a complete failure and maybe players might not even have missed it anyway.
This post also seems to somewhat corroborate with Treyarch’s Dan Bunting’s at that time, where he said that they never set out to create a “traditional campaign”. Based on this post and rumors of a 2v2 co-op campaign, it certainly doesn’t sound traditional to begin with.