He lost control of the film. Taken away probably. He was not calling the shots at the end either way. Perhaps it just was not working or atleast that seems to be the general consensus.

But it is not like he became an enemy or something it just got taken away.

It happens a lot now days.

And the music composer disappeared as well probably because of how much a re-write and liked what he had done and did not want to change what he had done.

On Godzilla Gilroy was also btought in on that film… But I still like Monsters also