Apart from being forced by VC ( because they are heavily invested in Amazon ) i could not understand why startup are choosing AWS at all.
S3 is very reliable, and I'm not aware of anything as reliable (we're talking eleven nines)/simple/cheap/distributed.
EC2 gives you the ability to manage instances in several continents with a dead simple API, which is unheard of anywhere else at this price.
ELB is very reliable, simple to use, and saves you hours of setup.
My point is that if you really wonder why anyone would want to use AWS, you might not be considering AWS, but only EC2, and in a very non-international way.
It's dead-simple to get started.
It's widely known. I can convince my CTO to use Amazon for cloud-hosting. He's never heard of Rackspace or Joylent - people who are serious about running their infrastructure on the cloud will research and know about them but for quick experimenting and companies just getting their feet wet, Amazon has a big edge here.