Because in that case, you need to specify the window's x, y, width and height.
Instead of using 'super', you need to use super(value_of_x, value_of_y, value_of_width, value_of_height)
Note that you need to specify each of that value on Window_Base child's
Window_Selectable is Window's_Base child, and as your window is child from Window_Selectable, the arguments are passed =)
If you still have any doubts about this, try to read the first lines of standard editor windows.
See you