The probable explanation for the NES Classic Edition controller cord’s short length

On the whole, people are pretty pleased with the NES Classic Edition – not withstanding stock issues. However, there is one complaint that’s been mentioned here and there. For some players, the controller’s cord is just too short.

Here’s an interesting thing about the controller: it packs in the same communication protocol as the Wiimote. Both use I2C, which has limitations with speed and length.

Had Nintendo gone with another communication protocol, it would have required new controllers just for the NES Classic Edition. On top of that, it wouldn’t have been compatible with the Wii U or Wii.

Source