That's a pretty good point HarryT, and probably seems a bit of bizarre logic. However, there were three main factors in buying the Kindle. In order of importance.
Price. I wouldn't pay over £100 for an ebook reader. I've just paid £68 for a refurbished Kindle, nothing else can match that, can it? (In addition, after-sales support, which Amazon tends to be pretty good at.)
Buying books. I only ever buy books online from Amazon, I guess buying ebooks is slightly different, but as much for convenience than anything else, there's no real reason to change.
Calibre. The fact that Calibre looks as if it makes converting formats for use on the Kindle so easy means it's not really much of an issue.