Both struggle due to the generational increase in budget and time traditional game developments require for both consoles. Making software for the 3DS takes similar resources as a Wii one... and Nintendo already has a history of droughts, the few releases Wii had during its last years was blamed on the 3DS. That's why they are merging their development for future generations. Just develop one Mario Kart that works on both, develop two mario's (2D and 3D) instead of four. They can also merge the two zelda's (2D handheld and 3D console) to release on both devices. Then the game releases doesn't become oversaturated as some people complained during the past few years.
And since they can support both devices with just one title of their main series, then they can dedicate their time to develop other genres or make new ips.
