Activision has made a statement the about lack of compatibility with the DSi and Band Hero's Drum Grip peripheral.
In short: there are more DS Lites than DSis out there.
This is what the company has stated: "For Band Hero
DS we chose to focus our development efforts on the primary DS platform, where millions of fans have already embraced the handheld innovation we brought to the Guitar Hero On Tour
"The new Drum Grip does not require the use of DS Lite's expansion port. The grip itself slips over the DS Lite replacing the directional arrows and buttons with 4 drum pad buttons that allow for a more natural, responsive and enjoyable play experience.
"While we will continue to innovate the portable music game experience and we are very excited about the emerging technologies available with the DSi, Band Hero is not compatible with DSi at this time."
Now, SPOnG doesn't work in manufacturing, but we can't imagine that making a slightly bigger rubber sheath so that Band Hero
could be sold for the DSi would be that much effort. Clearly, however, the DSi's installed base doesn't warrant that effort.More on Band Hero DS here