Although I wanted the white version of the Wii U console, the black one actually does have 32GB Flash memory, but what is it for? Is it for game play data save, so if you only play a few games, then probably 8GB might be enough?
The internal flash memory is only used for storage of things like saves and assuming digital downloads and the like. Apparently the only way to extend the memory of the Wii U is to use a USB storage device.
As to the second part of your question, unless a Nintendo Executive has made a public statement somewhere we can only guess as to why they made these two versions and went with USB over SD Card for storage.