Why can’t I tap and hold the Smile key to switch keyboards?

iOS 8 provides custom keyboards with only one method to switch keyboards, and it has no accommodation for tap-and-hold. It can only switch to the next available keyboard in the order you set in Settings > General > Keyboard > Keyboards.

We have a feature request on file with Apple. If you have friends in Cupertino, they can point their browsers at: rdar://problem/18313390