David Aaronovitch, of The Times Online, explains why, here:

In part I think that anti-Americanism is linked to a view of change as decline. The imagination is that dynamic capitalism, associated with the US, is destroying our authentic lives, with our own partly willing connivance. It is a continuing and - at the moment - constant narrative, uniting left and right conservatives, which will usually take in the 19th- century radical journalist William Cobbett (conveniently shorn of his anti-Semitism), and end with an expression of disgust over the Dome, the Olympics or Tesco. Just as bird flu is a disease from out of the East, runaway modernity is a scourge originating to the West.

So Barack Obama…will one day learn that there is no magical cure for the envy of others. What makes America the indispensable power (and even more indispensable in the era of the new China), is precisely what makes anti-Americanism inevitable.