Why would a field be missing from the Rules data selector?
I found the answer in the Commerce documentation: 


  This is due to the fact that Rules only knows about the default fields that we tell it should function in a manner similar to entity properties. In order to access additional fields, you have to do one of two things in your conditions:
  
  
  Use the Entity has field condition to ensure the field you want to access exists on the variable you want to use it for, or
  Use the Data comparison condition to check the type of the entity (i.e. product type) to gain access to every field on the entity.