The reason for this is the minimization of DLC and other such bullstuff on the Wii, and also the fact that it works perfectly with HDDs and flash drives as extra storage.
The Wii had a even smaller hard drive than the Wii U. Plus the Wii U is going to do way more DLC than the Wii, so I don't get where you are heading with this. Bigger built-in hard drives are
always better. You don't want to run out of space and be forced to buy external storage just so you can download more things. The PS3 could already use flash drives as external storage even before the Wii could, another case of how Nintendo just isn't keeping up with the times fast enough.