why not just use an SD card?

Because then you could buy any old SD card and Nintendo makes no money from you. This way Nintendo can sell you the same NAND flash except wrapped in a propriety Nintendo container with the multiple orders of magnitude price increase to pad the bottom line. They can also pursue you for civil or criminal charges if you attempt to break into the container of flash charging you with copyright infringement and attempted hacking.

Edit: ignore my post. Its not a Nintendo first party product.