The biggest reason Santa Monica Studio elected to conclude this era of Kratos' life was due to the scale of development. They didn't want to spend almost two decades developing games that tell a single story arc, given the development times of God of War (2018) and God of War Ragnarök. While God of War Ragnarök could've been split into two games, sticking with one game ensures the story isn't spread thin, which will make God of War Ragnarök as massive and as epic as possible.