Probably because you're visiting a normal town for the first time in the game.
It's because after dealing with immortal generals and inquisitors of Vlaakith, you are going to the circus, looking for clown body parts, solving a murder mystery that it takes one cast of Speak with Dead to solve, saving hairdressers from an assassin and overall dealing with what feels like lesser threats than in act 2. The fact that humor and comic relief (that was entirely absent from act 2) suddenly makes a comeback is also a factor.
However, the more I think about it, it's not act 3 that feels like it's from a different game, but act 2.