Short answer: Because each lines of code must be executed before moving on to the next one.
Explanation: In your example, you create an instance of myClass1 and on the next line add it to the Display list but before it is added it does run the code that is in its constructor and in that code there is a reference to stage which at this point is null because that instance is not yet on the Display List. So that's how it goes:
var classone:Class1 = new Class1();//all code in constructor runs
addChild(classone);//now classone has a valid stage