Because it was wonderful! I've talked to a lot of musical theatre folk who said things like "I didn't understand what was going on," "It didn't make any sense," and "I just didn't get it." Their problem, though, is that they have a narrow-minded view of what musical theatre should be, and their brain basically shuts down in protest when a show doesn't lined up with their definition, and then it doesn't make sense to them because their brain isn't working.*
Because the plot of American Idiot was not difficult to understand. And I really really mean that. The plot is super basic: three guy friends are in a band. They are disaffected with the state of the world and also the state of their lives. This is a very pomo 20th Century story. You have all read it and seen it probably 247 times. Two of them go to the big city (the third doesn't come because his girlfriend is pregnant). One of those two is unhappy in the big city and joins the army, thus "betraying" his friends and his previously professed socio-political beliefs. The third (our main character) remains disaffected, gets hooked on drugs, falls in love with a woman, and is basically confused. Eventually their lives deteriorate; eventually later they pick up the pieces and get to hang out together again, if not happy at least making lives for themselves. Easy easy shit to follow. Really!
And I don't mean that as criticism of the show, because the basic plot of the show is not the point. This is a rock musical. It's not about what happens to a few guys. It's about coping with the modern world; it's about becoming an adult; it's about living with disaffection. And there are a lot of ways to interpret these messages and the ways they are presented. I was actually afraid of disliking the show for the first 15 or 20 minutes: I was afraid it would not have legitimately human female characters; I was afraid it would fully embrace postmodernism and the fracturing of communities as a result of identity politics; I was afraid it would position the poor disaffected American white guy and his pain as the center of the universe. I should have given Green Day more credit, it turned out. Although it unfortunately remained a musical about guys, I was impressed by the well-rounded humanity of the gal side characters. It embraced community without conservatively leaning on the "standard" hetero nuclear family (the way neo-liberals in the media, and also Judd Apatow, habitually do). It recognized that the most sympathetic disaffection is about stuff going on that is bigger than your poor little while American male self - while reminding us that everyone's pain is real, whether or not it's worse than someone else's.
And there was more to think about and deconstruct, although I can't remember most of the details now because it's been three weeks or so since I saw it. But basically, it was great! And no, the songs don't always further communication among the characters and serve the linear development of the plot, the way we generally ask songs to do in contemporary musicals. The songs explore the inner lives of the characters and the greater themes of the show, in a frequently nonlinear, nonliteral fashion - which doesn't mean that they are stuck in without relevance to the plot or characters, or that they are not necessary to the telling of this story. What it means is that they are unfamiliar and confusing to people who take musical theatre seriously (which I do) but who in the process end up defining it in a highly focused and limiting way. (You know, I love Jason Robert Brown and Adam Guettel and Kerrigan and Lowdermilk too, and I'm excited about The Book of Mormon too, but I also recognize that they are not the be-all and end-all of contemporary musical theatre!) So American Idiot gets dismissed as fluffy or nonsensical or irrelevant or pretentious because it's ROCK music and it's ANGRY and it's NOT ABOUT THE PLOT (basically for the same reasons that Passing Strange was also dismissed among the gospel musical theatre crowd).
But, I will have you know, what it also is - is GREAT.
*Disclaimer: I adore many members of the gospel musical theatre crowd. I agree with many of their opinions about musical theatre. Many of them are dear friends of mine, and people whose opinions I often respect greatly. I just happen to absolutely passionately think that they have a disastrously limited vision in respect to well-crafted rock musicals.
Also, I believe that there are reasonable reasons a person might not like this show. They might not like the music, and that's fine. They might think that the plot doesn't support the message, and that's fine. They might dislike the characters, and that's fine. They might morally disagree with some or all of what they or I interpret as the messages of the show, and that's fine (well, it might mean that they are conservatives and therefore morally bankrupt, which is not really fine, but I would accept it as a legitimate reason not to like the show) (also, a person could probably politically dislike the show for non-conservative reasons, but I don't know anyone who does so I don't know what those reasons might be). There are, in fact, a lot of reasons a person might not like this show, and I am completely willing to talk about it. I am just not willing to talk about how the show "just doesn't make sense."