Typical Hierarchical inheritance in Java

Consider this below code snippet

public class SuperClass {
    public void move()
    {
        System.out.println("i am in super class");
    }
}
public class SubClass1 extends SuperClass{
    public void move()
    {
        System.out.println("i am in sub1 class");
    }

}
public class SubClass2  extends SuperClass {

    public void move()
    {
        System.out.println("i am in sub2 class");
    }

}

Now i am creating object like this.

public class program {
    public static void main(String[] args) {    
            SubClass1 obj = new SubClass2(); // Compile error - Type mismatch: cannot convert from SubClass2 to SubClass1
            obj.move();
    }
}

Why i can't do like this ? What stopping me to write this ??