3DS and Wii U are not symmetrical platforms at all.
The DS/3DS are really effectively just one vertical screen, seperated by a border that you can quickly look at. The 2DS even took that further and is literally just one screen with an arbitrary plastic border between it.
The Wii U is nothing like that. The screen is completely separate from the TV screen, to look at it, you basically have to stop playing, look down, and then look back up. It's a concept that just doesn't work very well to begin with not surprised a console based on this idea has been a massive failure.
Even when I play DS backwards compatibility games on Wii U, I usually have the screen mode set so that both screens are on the Wii U tablet and I'd bet that's probably how most people do it. I don't want to have to be looking up, then pause, then down, then back up to the TV. It's much more closer to the DS experience to have the screens side by side on the Wii U tablet screen even if it means they are horizontally side by side.
In any case for backwards compatibility just allow the screen to be held vertically. If you need to add "side wings" or a cradle type thing for physical controls, then so be it. It would just be a piece of plastic that would cost a few pennies to manufacture.
