I think Ginger is really on to something here, that the New Sincerity isn't just a response to contemporary poetry, but a perceived lack of sincerity in American culture.
Because I do think it is such an artistic and cultural touchstone, is this, at heart, a response to September 11? To our president's edit that "America is open for business?" Perhaps we're open for—to—more than that now, including the realization that life is too short to be a cool cucumber and not long enough to play clever games.
Within 30 days of September 11, the longest relationship I was in ended. People I knew got engaged and quickly married. People moved out of cities (or to new ones), quit jobs, started or changed families. It seems that maybe we take things a little bit more seriously now.