The WiiU gamepad has a lot of chips inside....which make sure only the signal from the console will be accepted.Without these chips (cough, ithis system is called "DRH") which stands for a communication-hub, you can't connect any device to the console & expect it to stream. Yes, both components (the WiiU console itself & the gamepad) have these components inside.
The WiiU gamepad does a lot of processing, you'd be astonished. Without it you couldn't even Update your console. Because the Gamepad itself contains the firmware-update-chip (32 mbytes, very big in size) as well.
So what would you update with any standard-pad when there's no firmware-update in the console itself? lol
A console with a failed firmware-update can't boot. The WiiU however is special & can boot wether you connect a gamepad to it or not (faulty firmware doesn't matter). This decission was done on purpose because many consoles/PCs these days get damaged/stop working after a firmware-update of the bios/uefi-bios.
The WiiU Gamepad's DRH also controls red/green/blue colours & adjusts the contents in your stream. E.g. if it detects a fast racing game, colour red is reduced & bitrate gets bigger.