One of the original purposes of the Curia (or what became the Curia) was to administer the Papal States.  Since the Papal States are a thing of the past, why is the Curia not a thing of the past.  The message from the Vatican seems to be, "We don't have the Papal States any more, but we are still sovereign and important, a force to be reckoned with in the secular world.   And our ever-growing bureaucracy proves it!  So there!"