The decition was not completelly unreasonable. It took 5 years to make the GoW2018 and another 5 for the Ragnarok. At some point during the second game the director started to think if they really want to spend 15 years of their life by making the trilogy so they decited to jam the two last games in to one.

I dont like how the game ended up, but i honestly understand why they cut the third game.