Barlog provides the reason in new interview
Santa Monica Studio’s Cory Barlog, director of God of War (2018) explains in a interview with Kaptain Kuba (via Youtube) why the Norse era God of War will consist of two entries only.
Barlog basically summarises the reason comes down to time, explaining "I think one of the most important reasons is the first game took five years. The second game, I don't know how long it's going to take but I'm just going to throw out that it's going to take a close to a similar time.”
He says did not want the story to be stretched out close to 15 years, stating “I feel like that's just too stretched out. I feel like we're asking too much to say the actual completion of that story taking that long... it just feels too long”.
God of War: Ragnarok will release sometime in 2022.