The why is summed up in part by the endorsement—"if only the real John McCain had been running"—and in part by last week's Conservatives for Obama, so-called Obamacon:
The biggest brigade in the Obamacon army consists of libertarians, furious with Mr Bush’s big-government conservatism, worried about his commitment to an open-ended "war on terror," and disgusted by his cavalier way with civil rights.
Of course, the endorsement should be no surprise. The Economist has a history of endorsing the other party: Governor Reagan in 1980, Governor Clinton in 1992, Senator Dole in 1996, Governor Bush in 2000, and Senator Kerry in 2004.
With this heady endorsement, the Illinois senator might just win.