Generally speaking, ports of games from a different platform are a poor comparison of the two platforms' capabilities. Each game is different, and each port has been specifically optimised for that particular title to run on its destination platform.
For example, Xenoblade Chronicles was specially modified and optimised to run on the N3DS, and these changes were specific to Xenoblade Chronicles only, and are not applicable to any other Wii title.
On the other hand, emulators must be able to handle multiple different games and therefore don't have the advantage of being dedicated to running just one game smoothly and reliably. This means that the requirements for an emulator to successfully run are vastly different to what it takes to make a port. That's why the argument that "If N3DS can run Xenoblade Chronicles, then why not any other Wii game?" is invalid. That's because the 3DS isn't actually running the Wii game, but a specially modified version designed to run on the 3DS hardware.
It's the same as when Super Mario 64 came out on the DS and people demanded that a full speed N64 emulator be made for the DS.