Diapers.com's customer focus is slightly different than Zappos - eventually at some point in time Diapers.com's present customers will no longer have any reason to continue shopping there (it's true for us today - we don't need anything from Diapers.com), whereas Zappos can potentially sell you shoes for a lifetime.
As an aside, I personally find purchases clothing (including shoes) online in the not-quite-right category. How can you ascertain the fit and aesthetic of a shirt or a shoe without actually holding it in your hand or trying it on?