Cost, control, certainty, per-machine performance, simplicity. You don't get "dynamically scale" for free just by using Amazon. So if you aren't going to put

Cost, control, certainty, per-machine performance, simplicity. You don't get "dynamically scale" for free just by using Amazon. So if you aren't going to put the up-front investment into building your system so it can scale quickly then it frequently doesn't make sense to use amazon as it is more expensive. Further, EC2 has a lot of uncool attributes, like low and inconsistent IO throughput both from machine to machine and to EBS, certain limits about the network configurations, and the fact that you have to go the "let it fail" route instead of trying to just have redundancy within a machine. "Let it fail" works for certain kinds of workloads and architectures and requires more complex software.