I think most felt that after Far Cry 3 it was difficult to top so I think 2 things happened with Far Cry 4; 1, they tweaked some of the core gameplay but mainly 2, they just released at a bad time. It was a decent game but not enough to grab you from the AAA games out at the time.
We've all had that feeling in our lives... 'you were good, just someone was a bit better on the day'.