This isn't surprising, but nor was it obvious. As like the NYT, the Economist also supplies a link to their past endorsements; unlike the NYT, they choose their preferences on the merits of the candidates and not according to who the Democratic candidate is. This makes them more credible.
Their reasoning is mostly convincing. Actually, it's quite similar to that of Leon Wieseltier, to whom I linked a few days ago. His language is the more poetic and compelling, they're more fair to John McCain. At the end of the day, after acknowledging that Obama is a gamble that could go wrong, they wholeheartedly support him and hope for the best.
A plausible position. And, given that he's about to win, we should all be hoping he's up to the job. The world needs a strong and confident America.