Because it was hopeful and positive, even joyous, rather than morose.
The standard comparisons of the past four days have been to Ronald Reagan after the Challenger disaster and Bill Clinton after Oklahoma City. Tonight's speech matched those as a demonstration of "head of state" presence, and far exceeded them as oratory -- while being completely different in tone and nature. They, in retrospect, were mainly -- and effectively -- designed to note tragic loss. Obama turned this into a celebration -- of the people who were killed, of the values they lived by, and of the way their example could bring out the better in all of us and in our country.
That is to Obama's imaginative credit. (Even as the event began, I was wondering how he would find a way to match to somber tone of Reagan and Clinton.) More later, but a performance to remember -- this will be, along with his 2004 Convention speech and his March, 2008 "meaning of race" speech in Philadelphia, one of the speeches he is lastingly known for -- and to add to the list of daunting political/oratorical challenges Obama has not merely met but mastered.
UPDATE: I have turned off the TV after listening to various pundits moan about the "unseemly" cheers from the audience, the length of the speech, and so on. Here are two reader messages just now that correspond to the way I heard it: