Amazon also has their huge ecosystem of content that B&N just can't match. Barnes & Noble's specialty is books, and that's about it. Amazon offers so much more than that, especially with their Amazon Prime service.
Amazon's huge ecosystem (books, music, movies, Amazon Appstore, etc.) is ALMOST as extensive as Apple's. Yeah you can spout the numbers game, claiming that Apple has "over 700,000 apps", but Amazon is no slouch either.