Nintendo's inability to embrace the growing "core" market is what's caused their decline. They don't appeal to third parties, their online infrastructure is lackluster at best and almost their entire 1st party line up is "kid friendly". As people often say, you buy a Nintendo home console for Nintendo games. If you aren't fond of their 1st party line up then their consoles are rather unattractive compared to their competition.
The Wii was an anomaly since it appealed to a previously non explored market. Once the "casual" market had moved on the Wii's sales started falling back towards that previous trend (one that the WiiU is now following).
Really though, the only trend here is Nintendo's unwillingness to embrace what the majority of the core market wants.
