I don't know that it would have been possible to release a Wii U gamepad device for the Wii, the Wii is built on technology from a decade ago so asking it to render two screens at once while transmitting that data wirelessly may have been two much for that system. Nintento have already admitted that the framerate of the Wii U will be halved when using two gamepads at the same time.
Nintendo, unlike MS and Sony, makes most of their money from hardware sales, they don't make much money from software because casual gamers don't purchase games for the Wii, they're happy with Wii Sports, Wii Play, Wii Fit, and so on. But the Wii pretty much reached market saturation as everyone and their grandparents already owned one, so Nintendo's revenue crashed. They've been in the red for 2 years now (I think) because of the drop-off in Wii and DS sales, they needed to get another console out the door to try and return to the boom days of 07/08. Sadly for them, the 3DS's launch wasn't as successful as they hoped, and the Wii U looks to be facing similar apathy from the general public, so they're going to be in a bad place if this launch doesn't go well for them.