Today, the Chicago Sun-Times wrote about the effect of Pitchfork on small albums. Their prime example is Arcade Fire's Funeral, an album that the notoriously stingy Pitchfork absolutely loved. After giving it a 9.7 out of 10, Arcade Fire's record label, Touch and Go, couldn't "keep stock in the building, stores were [sold] out. We were re-pressing them as fast as we could, but the demand was so great that we couldn't keep up." They've now sold around 28,000 copies, which is pretty amazing.
Personally, it seems the reason this can happen is because Pitchfork is one of the only places online for reputable record reviews. The only other two sites that seem to have any real cred are Tiny Mix Tapes and Pop Matters (If you know others, please share in the comments). On top of having only two competitors, TMT showed up well after Pitchfork became successful and Pop Matters doesn't appeal to quite the same audience. So, it's fairly obvious why Pitchfork wields so much power -- they have no real competitors.
Instead of asking why Pitchfork has such power we should ask why there aren't more music sites that command the same amount of respect. One could argue that Pitchfork's reviews are incredibly well-written, but I have a few dozen friends who would quickly disagree.
I don't have the answer, but would guess it has something to do with their ability to post highly regularly and that they're still around after ten years (according to their advertising page). Can you figure out why they're so dominant?