why static variable not used in instance?

A static variable refers to its containing class or struct as a whole and not to any particular instance of it. Consequently, there is just one copy of each static variable which persists throughout the lifetime of the application.

Whilst instance members of the class or struct can refer to static variables, static members cannot refer to instance variables unless, exceptionally, you pass them to the static method as parameters. 

Dear !
Check this.

http://www.coderanch.com/t/528608/java/java/Difference-Static-Instance-variable