I propose the following hypothesis as a partial explanation for why something as trivial as the "Itisasitwas Intrigue" (which includes the argument that it is not trivial) is getting the play it is:
The story, such as it is, lies at the intersection of the two most important stories in the history of the world.
The most important story in the history of the world are the life, death, and resurrection of Jesus. The second most important story in the history of the world is the most important story's sequel: the story of the Church. Since they are the two most important stories in the history of the world, who gets to tell them and how they are told are matters of grave importance.
The debate over The Passion of the Christ is a part of the debate over who gets to tell the story of the death of Jesus, and how they get to tell it. That, I think, is clear enough (though others don't share my opinion that it is a very small part of the larger debate).
The debate over the Pope's reaction to the movie is, I suggest, a part of the debate over who gets to tell the story of the Church, and how they get to tell it. One version of the story -- the "Vatican ≡ bad" version -- serves to sever the institutional Church from the metaphysical reality of the Mystical Body of Christ. Thus, both people who want the story of the Church told this way and people who are adamantly opposed to the story of the Church being told this way have a lot to say about it.
So this fleeting little story, by itself a trifling part of the two most important stories in the world, is so to speak borrowing importance from the larger and enduring stories, simply by being at the spot where people have chosen to stand for their debate.