Because it's pointless and Relic doesn’t care, CoH2 was abandoned three years ago, even Relic thanks were made not Relic. I think Italy should have come from Britain DLC together, this is a very stupid unequal number of opposing factions. But now this is just an excuse to sell you all the same British and US for the third time.
I mean the Italian faction makes sense to implement, but with the focus on the factions you could argue, that the Italian faction just didn't quite fit in. First we had Ost vs Soviets with a clear focus on the Eastern Front. Italia had no real place in the vanilla game. Out came the Western Fronts DLC which mainly focused on the Ardennes Campaign which clearly was USF vs Wehrmacht (OKW). You could argue, that Brits where then in fact an addition to the Western Front Armies because they are focused around the late war British Army which mostly fought in the West after Normandy.
But now you might ask: Why didn't the Italian faction bolster the Eastern Front part of the game? Answer: It was never planned and didn't really make sense because you could then argue, that a mixed "Auxilliary"-faction would have made more sense because the Italians were just a part of many foreign Armies fighting under the German regime. Stand-Alone Italian would only make real sense in a possible COH3 if it would be focused around the Mediteranean campaigns