oh, and price.
That's the big advantage of the Kindle. Buying books and getting them on to your reader is a simple, one-hit process. And Whispernet means you don't even need WiFi.
Buying books for the Sony devices has always been a right mare - remember, the older versions had no wireless, and the protocol from PC to device is simple file transfer. So, buying a book is a multi-step process that all but the most dedicated would give up on and return the device. Don't get me started on the content-provider-specified bowl of sick that Adobe Digital Editions is.