Nintendo lost the core around the year 2000.
About the time when people got tired of playing GoldenEye for the 10000th time and realized there were about 10x more games on the Playstation, even the most ardent NES/Super NES fans at that point jumped ship and bought a Playstation as their primary console.
By the time the PS2 had come out and the XBox had arrived, I think Nintendo in effect kind of just gave up even trying all that hard for that crowd. When Resident Evil exclusivity couldn't salvage the GameCube, I think Nintendo realized it would take a lot of effort to win this audience back and decided to simply not even bother. Cut ties with Factor 5/Silicon Knights/etc. and in large part they cut back on third party deals collaborations too (ie: Metal Gear Solid: Twin Snakes, F-Zero GX, Mario in NBA Street, Capcom 5, etc.).
That was the whole point of the Wii, to go in a dramatically different direction and chase different consumer bases (ie: women, fitness junkies, seniors, hipsters/lapsed gamers). That's where the Wii brought in new customers for Nintendo.
The problem is Nintendo didn't know how to keep the casual audience interested either, and has been watching in horror as much of that crowd has gravitated to cheap smartphone and tablet gaming. So now they're kinda trying to step back towards the core, while still trying to be open to casuals, but they're far apart from both groups right now.
