I think it's because there's no trust in the big names anymore. Most of their "reviews" seem to be largely written by the game studio. How many times can you see IGN gush about how awesome a game is before launch and then when you get your hands on it, is a buggy boring mess before you stop checking IGN?
A lot of it is on the publishers for their early review copy practices. Big media outlets will get early review codes for the games so they can be among the first to get a review out and net the traffic. But the publishers want good publicity in return for the early access codes, and reviewers that don't play ball can find themselves on an industry wide blacklist from receiving them in the future.
So the best, least biased reviews you can find are going to be 2-3 days post release, and not from someone who is large enough to get free review codes. I never trust pre-release reviews.
