The Wii released at the right time and appealed to a previously untapped demographic, casual (in the non-pejorative sense of the word) gamers with its focus on motion controls. Unfortunately for Nintendo, that market isn't particularly loyal to the brand nor do they really care about what happens next, so despite huge sales, the rate of those sales slowed over the years. As for the WiiU, there's a combination of factors that led to the lukewarm sales. The casual gamers that were the backbone of the Wii's success didn't stick around. Hardcore gamers were alienated by Nintendo. The name itself implies it's an add-on for the Wii instead of a brand new game console. Lack of third party support. Piss poor marketing. The tablet controller is unwieldy. It's specs are weak and merely caught up to the 360 and PS3, instead of leading the pack as Nintendo used to do. And so on.