Reviews are almost always put out just a few days before.
The huge bulk of them, yeah. But you still usually see a handful of reviews a week or so beforehand.
I wonder if it might have something to do with the controversial political themes in the movie we've heard about. The studio might now want a bunch of bloggers and news commentators making a big fuss and getting morally outraged over it right before the movie comes out...
The studio could have a huge embargo/gag order on the reviews. Anyone who saw the previews likely signed paperwork to the effect and if they break the embargo they forfeit their first born child to the Firegods of Warner, or they'll be sued into oblivion.
Either way breaking the embargo/gag order wouldn't be a smart move for a professional journalist or publication (even the net-based ones.) Studios generally take this kind of shit very, very seriously.