Really close to why I chose Amazon as my main bookstore, and originally as my device type; in other words went with the Kindle.

Once upon a time I decided to try out the new Sony eReaders as they looked really cool with more bells and whistles, but after several months of frustration and disappointment with the Sony eBook Store, I gave up and returned to Amazon. Amazon built a great eBook store, and that made all the difference to me at the time, especially since I rarely read fiction. I read histories and science related books mostly, and only Amazon had much choice in those genres back then.

Later I decided to do my reading on non-eInk devices like my smartphone and tablets. So the Kindle was not even a factor any more, but the bookstore definitely was. I once tried Nook and B&N, but that was not good either. And I also tried Kobo, but they couldn't get their FW working right and their store was lackluster too.

ETA: I also started reading eBooks on a Palm Pilot. Pretty cool for a day and time when very few people knew you could do that. Most people thought Palm Pilots only had stuff like appointment calendars and what not, so I could pull my Pilot out of my pocket during extremely boring company meetings and tune out their nonsense by reading. These were mostly pep rally type of meetings by a company that was quickly spiraling into oblivion. And of course, the ability to hold a small device in one hand and read as opposed to trying to balance a paperback or hard cover printed book, keep the darn thing open, and then still have that cruddy curve to the pages. Yep, a flat screen and easy handling sold me on eBooks very quickly.