A minefield is only active when the naval base is controlled by either the original controller or another power on the same side. Normally, the original controller controls a naval base if its territory is contested (unless its capital is controlled by an enemy power). However, since Italy has been eliminated from the game, it cannot control anything, and Italian territories no longer have an original controller.
As a result of all this, as long as the territory is contested, neither side controls the naval base. If a power on either side gains control of Rome, it will also control its naval base. However, with Italy out of the game, no other power is on its side, so neither side can activate the minefield.