Some more items:
o Liquidity of engineering talent - thousands of engineers know how to immediately work with the AWS infrastructure
o Design Patterns - They are constantly being updated, discussed, improved upon. People share how to work around AWS issues.
o Commitment to reducing prices - AWS is constantly dropping the prices for everything.
o Commitment to increasing the number of services - AWS is constantly releasing new and useful services on their infrastructure.
o Likelihood of being in business in 3 years. AWS isn't going anywhere. You can rely on them being around.
o Really good post mortems. Each time they fail, they do pretty transparently, and deliver a really painful post mortem that encourages me to believe they are committed to constantly improving.
o Multiple Availability and Region zones. I can, with very little effort, make my services available in redundant regions. I don't think there has ever been a multi-region failure with AWS, and, at most, it's only happened a couple times in their history. Multi-Availability zone failures have happened (and Amazon has apologized - that shouldn't happen) - but, it's < half a dozen times.
o Market Leader Advantage - The market leader tends to win. Microsoft for Desktops, Oracle for Databases, Linux for simple Web/Database/PHP stacks. This isn't always the case (Juniper can beat Cisco in a lot of places) but you know Amazon will be a Top-3 contender for at least the next 5 years.
I realize a lot of these are "Business" reasons, which is incredibly frustrating for people who are comfortable with hosting bare metal, and realize that, given just half the money, they could go into a CoLo, rent a cabinet, buy some servers, routers, firewalls, storage - install it, and return twice the performance that they would get on AWS.
But quite often, the business argument is a more important driver for CxO people making decisions (and assigning budget) than technical reasons.
And, with all that said - I've been in about a dozen Tier-1 data centers in the last 15 years, and, with one exception, every one of them has had multiple sustained outages. (The exception has been Switch SuperNap in Las Vegas - 3 years and no major outage.) - So, failures are part of the business. I'm not expecting Amazon to eliminate them, so it doesn't disappoint me when they happen.