Look's like it's the same reason why they skipped the Wii initially. But they are right, WiiU Wii owners (86%, see below) already purchased the RB1+2+Beatles+Lego. WiiU is already backwards compatible, so the demographic can already play the games, hence why we don't need a WiiU Rock Band 4. These games are about music and not graphics, right?
I just skimmed through the first responses, and I saw people talking demographics.
There was a study done recently released by Nielsen which showed some numbers.
http://gamrconnect.vgchartz.com/thread.php?id=198827

| Nintendo has tapped into its loyal fan base with its latest system—86% of Wii U owners are prior Wii owners. Consumers appreciate the fun-factor, kid-friendliness, and value of Wii U. Not far behind, three of four Microsoft Xbox One owners are previous Xbox 360 owners. And this is understandable, as these consumers indicate their top reason for purchasing an Xbox One was the brand. Meanwhile, PlayStation 4 has shown an ability to capture both previous PlayStation 3 owners, as well as competitive seventh gen users, thanks to its superior resolution, Blu-ray player and game library according to purchasers. |