Okay, not an exhaustive list, there are many many reasons. But as with other great companies, it's often the negative experiences with their competitors that highlight their strengths.
I've bought a lot of items from The Impossible Project, a company started by some ex-Polaroid employees to try to continue producing instant film for traditional Polaroid cameras. I own a few Polaroid cameras, I love the beautiful-ugly analog quality of the photos they produce (even after it was co-opted by hipster culture), and I was thrilled that someone would fight to keep the film in production.
In the craziness of moving to a new city and starting a company, I lost track of one order I placed with The Impossible Project for two small items. It popped into my head the other day like random things often do, in that "Remembrances of Things Past" way, and I realized I'd never received the items. I went online to check the shipping status, and it was marked as delivered to our office a few weeks ago.
I wrote in to report the shipment missing, and a customer service replied that the company was in an awkward position because the shipment was reported as delivered, so the best they could do is give me a store credit for the price of the shipment less shipping.
A totally fair and reasonable offer. But from my perspective, I'm now down the shipping cost on that order, and it makes me a bit sad to feel distrusted.
This is after ordering a lot of their early test films, some of which came from failed test batches that produced unusable photos. To their credit, they offered ways to ship back the bad film to get replacement film, but the overhead of packaging and shipping up defective products is its own hassle and cost, and I'm almost certain there are some lemons among the batches of film I purcased from them but haven't used yet. I don't mind supporting small companies that are trying to do good things, and the internet and web have created an entire class of entitled, self-important customers who feel aggrieved even when free products don't serve their every whim. But The Impossible Project's products aren't cheap, and I've spent a lot of money with them, so this botched transaction with them feels like a cold reality check about my importance as a customer.
But perhaps it's just Amazon that's spoiled me. I've never had Amazon question any order I've reported as missing. Amazon will ship a replacement order for any damaged or missing item, no questions asked. Once, I reported an order for a DVD as not having been delivered and so they shipped me a replacement immediately. Then the original shipment finally showed up a week later, and now I had two copies of the same product, so I emailed them and asked if they wanted the original back. They said it was okay, just keep them both and save yourself the hassle of shipping the original back.
Amazon competes for your business for life, while other companies compete transaction to transaction. When I hit the 1-click order button at Amazon.com (and I do that a lot, at least twice a week), I do so with zero doubt that I'll get anything less than full satisfaction.
When I was at Amazon.com, the whole company was fixated on eliminating two of the most severe psychological roadblocks to ordering online: paying for shipping and worrying about the cost/hassle if the shipment went bad for some reason. They've effectively cut both of those issues down to size, the former with Super Saver Shipping and Amazon Prime, and the second with their "no questions asked" return/replacement/exchange policies.
And that is why for millions of customers, Amazon.com evolved from a convenient way to shop for long tail items to the preferred way to purchase anything and everything.