This has to do with the same thing as what I answered in your other topic. An instance of a non-static nested class is associated with an instance of its enclosing class.
The body of a non-static nested class is not a static scope, and therefore you can't have static members in there. In other words, the member variables and methods in the non-static nested class are relative to an instance of the enclosing class - they are not really at class level (where you could use static). It's a bit hard to explain, but if you've understood my answer in your other topic, I hope you'll understand this as well.