N64 was the last "traditional" Nintendo console (ie: not relying on casual gamers) to do well in the US. The N64 shipped 21.9 million in the Americas versus 12.9 million for the GameCube ... so that was quite a hefty drop off.
Nintendo giving away the FPS market by not making a proper successor to GoldenEye/Perfect Dark hurt them big time.
GameCube looks good versus the Wii U, but that probably speaks more to how horribly the Wii U is doing.