Bad optimization. It was the same with Far Cry 3 on last gen consoles. Ubisoft didn't respect the performance offered by the consoles. As a result the game ran like a wet potato.The question is: Is this because of bad optimization or is the game just that taxing?
Developers need to understand what hardware is inside the consoles. And the respect the limitations. It's not difficult. If the game runs like shit, then remove some bells and whistles.
And I'm not even a game developer.