[jtsnow]

In addition to supporting GameCube discs, the Wii had physical ports for plugging in GameCube controllers and memory cards. So, not much of a stretch.

[user_of_the_wek]

The Wii Hardware was also basically a beefed up GameCube. Plus the Wiimote.

[extraduder_ire]

They even released a version of the wii without the gamecube ports or compatability (before the wii mini) which immediately supports gamecube games again if you solder the ports back on.

[scrame]

yeah and the DS had a GBA cartridge slot.