I think a lot of it has to do with the fact that shoes fit EVERYONE. Yes, we all have our shoes sizes. But you don't look "fat" or "flat" or "shapeless" in shoes. So, women can indulge in a fun, beautiful purchase without the perils of body insecurity.
Here is actually a good link comparing different online shoe companies, on the topic...
http://factoidz.com/online-shoe-stores-review-of-the-top-online-shoe-stores/
