consoles wars is one of the biggest reason imo.
A lot of sites have begun to favor certain opinions and certain demographics. You can easily go to a website, and quickly see a bias in either the site itself or in the forums to which much of that site is dedicated to.
Some strange things tend to happen. Look at the games that are backed by valve. Left 4 dead is probably one of the most average games of last generation. It was short, was horrible when played alone (ie, no dedicated single player), lacked content and variety, had practically no story (though what was there was nicely told through the safe houses). The online multiplayer was good, but even then it was generally weak in content. Had any other developer backed the game, it would've been universally panned, but because it was valve, and because of the general love for valve as a company, it received completely un-deserving award and recognition.
The problem isn't the bias to valve, but to valve's philosophy on game design. You get certain puritans and elitists that will completely ignore shortcomings of titles simply due to pedigree. Mario galaxy is another one, it's a fantastic game, but around the same time mod nation racer came out on the ps3. Now, here's the glaring issue, on gametrailers, they scored a story for mod nation racers, yet for mario galaxy they completely ignored the story. Why? because the philosophy of the game-makers and its pedigree. No one expects mario games to have a story, but that doesn't make it fair to ignore that aspect entirely, and expect a community driven kart racing game to have one (just because it's a new ip/developer). As a result we get inherently skewed reviews.
Another reason is the lack of standardization across game reviews. I'm not saying that everything is equal, a lot of things in games are subjective, but even then, categories need to be prooperly expressed and games need to be compared to their counter-parts not to some moving goal post, or flavour of the month, or popular opinion.