Why did President Obama do so badly in the first debate?

An article in The New Yorker quotes Laurence Tribe, who mentored Obama at Harvard Law School, responding to the debate:

“Although I would have been happier with a more aggressive debate performance by the President, I’ve had to remind myself that Barack Obama’s instincts and talents have never included going for an opponent’s jugular. That’s just not who he is or ever has been.”

Whether Obama's "instincts" are to go for the jugular might be impossible to know. But he seems to be capable of going for the jugular — when he's motivated to do so.

Remember when he said this?

While I was working on those streets watching those folks see their jobs shipped overseas, you were a corporate lawyer sitting on the board of Walmart!

I'm surprised that in the whole New Yorker article, which is all about what kind of a debater Obama is, there isn't a single mention of Hillary Clinton.