God of War is a massive game but, according to Director Cory Barlog, it was originally planned to be even bigger, boasting several additional boss fights. But crafting a truly epic boss battle requires a tremendous amount of resources, which Santa Monica Studio decided would be better spent on making the game they had, with its more limited number of boss battles, the best it could be. According to Barlog...
Y'know, we had so many more, it was a much more ambitious, crazier game. As you go through development, you start realizing, no, it's too big, we can't do this. One boss takes like 30 developers a year and a half. It's an absolutely massive scale when you really consider it.
The thing about the God of War series is that it has always been an over-the-top action romp with as many Greek gods crammed into each entry as Olympus could churn out. Now that Kratos has moved into the land of Norse mythology, though, his motivations have shifted. He's no longer looking to kill every god under the sun with his bare hands. He just wants to live in peace and help his son fulfill the final wish of his dying mother.
So for once, Kratos isn't looking for trouble. He's actively trying to avoid the gods, in fact. But that doesn't mean all of the gods are trying to avoid him. The result of that is less focus on massive boss battles and more focus on things like exploration, puzzle solving and the like. Depending on what you count as a flat-out boss battle, there are only a handful of major encounters sprinkled throughout the game. There are plenty of miniboss fights, though, with Kratos and his boy taking on hulking trolls, an optional collection of Valkyrie and the like.
Given Barlog's explanation of how much effort has to go into crafting a boss fight, it makes sense that the team would decide to throttle back on those massive encounters. They were already taking on a rather huge project simply by taking the series in a number of new directions, so piling on a bunch of additional encounters with Norse deities seems like an unneeded burden. While some folks were bummed that the new God of War only had Kratos interact with a handful of members of the Norse pantheon, it certainly served the story's themes well.
Also, given the basically guaranteed sequel, you could probably view last year's outing as Kratos' handshake with a world he's only just now truly becoming a part of. Now that he's been forcefully thrust into the fray, maybe the next God of War will boast more of those boss battles that had to be cut from the original game.