It gets a 6 because although everything is done very well, there is little new to speak of.
The basic third-person combat/cover mechanic is heavily used, and games featuring massive sprawling cities are now quite common.
The story and setting will feel familiar to anyone who has watched a Godfather movie, or any other similar franchises - Goodfellas, Scarface, Donnie Brasco.
Also, the entire concept of running around a big city, steeling cars, evading the cops etc has been done.
In fact, pretty much everything in Mafia II has been done before.
The 6 is meant to communicate this - don't expect to see anything you haven't seen before.
Does that help?