Because a lot of people have smaller bathrooms that don't have the space to give up to anything that sits on a shelf. A small bathroom, small sink area... where do you put speakers? Where do you put the main component? There are different ways to do it, sure, but this one is pretty silly, yet elegant. It takes up no more space in the bathroom and adds a functionality that many people may appreciate.
WHY put in an iPod Hi-Fi or equivalent when there is a better solution in the iPot?