Are you a Wii owner that's just itching to get your hands on Rock Band for your beloved console? I can't blame you, as the game is fun as hell. However, it won't be quite the same experience that is found on the 360 or PS3 due to the lack of downloadable content. When asked about this decision, Rob Kay, design director at Harmonix pointed the finger squarely at Nintendo.
Come on Nintendo, we need a hard drive. That's what we want. The whole problem is there's nowhere to store it. If the platform could do it, we'd jump on it", he continued. "It's something that we championed to Nintendo, that we'd like to do it. Who knows what will happen down the line, I don't know what's coming down the line, but that's the reason there's no DLC in Wii Rock Band
Honestly, it makes perfect sense. There's no reason to spend all of the time building in the DLC functionality if most users won't be able to really take advantage of it. Sure, you could buy a big SD card, but it's hard to expect all of your customers to go out and buy something extra like that. At least he left it open-ended for the future.