In the words of project lead Chris Foster, as noticed by 1UP:
One of the reasons we could do open-ended DLC for The Beatles [Rock Band] is that it's all tiny clips, and we could just reconfigure them for different songs as needed... With [Green Day: Rock Band] we're really creating sculpted experiences for every single song; there's an animation that plays all the way through, pretty much. That's something that's not really practical to do open-ended.
Packing forty-seven tunes on-disc, Green Day: Rock Band launches worldwide in PlayStation 3, Xbox 360 and Wii form on June 8, 2010. Those that buy on PlayStation 3 or Xbox 360 will also be able to export the included songs for use in Rock Band, Rock Band 2 or the upcoming Rock Band 3, provided that they either a) pony up the $10 export fee, b) pre-order the game from GameStop or c) buy the deluxe "Plus" edition.