Sometimes, an availability zone for a particular region is missing during server creation....
From AWS Representative:
As Availability Zones grow over time and our ability to continue to expand them can become constrained. In these scenarios, we will occasionally remove the constrained zone from the list of options for new customers so that new customers do not accidentally start to build up their infrastructure in an Availability Zone where they will have less ability to expand. This means that occasionally, different customers will see a different number of Availability Zones in a particular Region.