Sony? Nook? Ipad? Sure selection helps Amazon but they also have a very good product and they are ahead in the game than any of the other alternatives that i know of.
I really can't speak of today, but less than 2 years ago when I was in the market for one, both the Sony and the Nook were well ahead of the Kindle.
The Kindle did not have a touch sensitive display. When you read news ePubs like I do, it's almost useless.
The Kindle did not support ePubs (not sure it does even now). There goes the option to buy books from different stores, or the option to find a better deal. Or the option to check out books from the library (in those days, none of the libraries I checked supported the Kindle - only in the past year did my local one add Kindle support). I think there were other formats the Kindle did not support which many other ereaders did. If it's hard for me to buy epubs from other stores on today's Kindle, then it's still a crappy option.
At the time, the Kindle's support for PDFs was poor. The others weren't that much better, to be honest, but Kindle usually ended last when it came to handling PDFs.
If I could find the notes I had written comparing some 5-6 different ereader brands, I could probably list more deficiencies. Sure, one or two of the brands were worse than the Kindle, but the Sony and the Nook (in those days) were better. Today may be a different story (the Sony is ridiculously expensive).
The Kindle started off with a bang. They just failed to stay ahead of the competition for a while and let others catch up.