Long story short: if we could have, we would have. Even the best selling RBN tracks on the 360 weren't finding a sizable enough audience on the Wii. The drop is too significant, in the number of Wii users that are connected, the number of Wii users buying DLC, and the number of Wii users willing to explore the RBN offerings.
We didn't discontinue RBN releases on the Wii because we are mean or we want to deny those players content, it's because it's not viable given the amount of work required compared to the level of support it received. There's no sinister alternative motivation, it's honestly that simple.