I shouldn't be surprised that this show is rated so high. First, it's based on a video game by the same title. Second, is is perfectly timed to coincide with the end of The Walking Dead.

Though I've only seen the first 3 episodes, it's not very hard to guess where the series is going. Man and girl traverse zombified United States and since she is immune she is the only person who can save mankind.

In case you forgot, the comic book and TV series Y: The Last Man already covered the immunity plot, and the walking Dead literally beat to death the zombie apocalypse.

While I understand those that have played the game are naturally more inclined to like this show, for the rest of the audience it's just more of the same.