It's not about the powerfulness, it's about the pixel count and bandwidth!
The 3DS has two 400x240 screens (2*96,000 = 192,000 pixels) for the 3D-display + the 320x240 TouchScreen (76,800 pixels) = 268,800 pixels. Compared to WiiU's 921,600 pixels in 720p or 2,073,600 in 1080p.
One pixel in RGBa is stored in 32bits (4 bytes), so we've got to multiply pixelcount by 4, which leaves us with:
3DS (w/ 1 3D screen): 691,200 bytes (~675kB) * 60fps = 39.56MB/s * 60 = 2,373.6MB/min (2.32GB/min)
3DS (w/ 2 3D screens): 1,075,200 bytes (1050kB) * 60fps = 61.52MB/s * 60 = 3,691.41MB/min (3,61GB/min)
WiiU 720p: 3,686,400 bytes (~3,600kB) * 60fps = 210MB/s * 60 = 12600MB/min (12GB/min)
WiiU 1080p: 8,294,400 bytes (~8,100kB) * 60fps = 474MB/s * 60 = 28440MB/min (27GB/min)
WiiU 480p(GamePad): 1,639,680 bytes (~1601kB) * 60fps = 96MB/s * 60 = 5760MB/min (5GB/min)
The WiiU's WLAN Adapter struggles with getting stable 4MB/s already, so getting raw HD Video via WLAN is nearly impossible!
Maybe one day you can connect your PC to the WiiU's Antennas that usually connect to the Gamepad, but that's loooooong in the future!
Edit: I hope, everything's fixed now! It's 10pm here, I need sleep!