Just an observation — & one that may well get me into some trouble, but what the hell (to paraphrase the great Tony Rolletti) if we can't get into trouble in a blog, where can we get into trouble? — is that the most successful genre for American operas seems to be the pageant. Treemonisha, Four Saints in Three Acts, Porgy & Bess, The Mother of Us All, Einstein on the Beach, Nixon in China, Satyagraha, Akhnetan, Europeras 1 & 2 ... are all pageants, a form that may have elevated roots in the English Masque but is most familiar to Americans through the grade school and church and boardwalk and award show and political convention and half-time show spectacles that are a formative part of our shared experience. I suspect that having pageantry as the default setting for the "serious" musical theatre is due in no small part to the ability of film and television to handle dramatic narrative and intimate scenes well, as well as the capacity for verisimo which the screen media share with the spoken theatre.* But mostly, our operas are pageants, because pageantry is the habitual means for marking a remembered event as big, serious, and significant.
* Robert Ashley's operas, some explicitly made television, have an vocal intimacy and (radio/television not 12-tone) serial shape that are the antithesis of the pageant ideal; I take this to be the rule-proving exception. (See this post on Ashley and R. Kelly).