I bought my wife a first generation kindle when they first came out and we've been hooked ever since. Initially we used to fight over the single kindle in the house, but then when Amazon released kindle as an app for IOS, a brilliant move IMHO, that really caused it to take off in our house. The convenience of being able to purchase a new book at any given time, being able to read it across multiple devices, and Amazon's aggressive conversion of books to e-format has meant a LOT more books are being read in my house while at the same time a lot LESS trips to the local Borders are being made (I'm kinda sorry to admit I'm one of the people helping to kill the once proud book and music chain).
The other side of the coin is the proliferation of self-published books in the Kindle (and NOOK) marketplaces. As a self published author myself, I can attest that Amazon makes it super convenient to get ones own work published on their marketplace. This further adds to Amazon's growing e-catalog as well as gives exposure to very good writers who might otherwise never be seen outside of their own word processors.
The two combined, along with a lot of smart marketing by Amazon, mean that I personally find the succession of e-books to the sales throne to not only be non-surprising, but to be something I wouldn't have thought to be anything other then a forgone conclusion.