Because reviewers tend to be very critical of movie-based games, particularly when they don't bring anything new to the table.
The game doesn't really do anything to stand out from the crowd. It's your standard first-person shooter with relatively linear paths, a basic weapon selection, uninspired graphics, and very little variety in the enemies you face. It doesn't really follow the events in the movie as it serves more as a prequel than anything else.
If you're a zombie fan, though, it's quite a bit of fun as it really makes you feel like you're in one of Romero's movies. And where this game shines is in multiplayer.
There are your standard deathmatch and team deathmatch modes, but there's also this cool little mode called survivor. Basically, it's four players working together against waves of zombies that get increasingly tougher and more numerous as you progress. The levels are pretty limited size-wise, and there is usually a very limited amount of weaponry and ammunition lying around. Each of us would end up specializing in one specific weapon type and would cover each other in order to progress. There's nothing like backing up into a wall while a zombie bears down on you, only to watch its head explode inches from your face due to your buddy capping it with a headshot from the revolver.
We found survivor mode so much fun that we'd set up deathmatches, which allowed us to have 8 players and much larger maps, but then we'd agree not to attack each other and only concentrate on the constantly respawning zombies that populate that mode. It was a blast, and probably one of the most fun experiences I've ever had on Xbox live. Well, next to Crackdown.