Because unless you apply early war concept(M3 Lee heaviest allied tank, that awkward bulky thing with short gun or MAYBE P3 as heaviest axis tank), then these factions will not be able to compete in any matter in late game.
No, anti tank guns will not be relevant and everyone, axis included would have their own PTRS for infantry AT.
It could be done, but its not going to feature literally anything WW2 iconic.
So because Italy had no super heavy tank like the Maus they are not suitable for a competitive rts game?
Jeah. That is great. Eliminate a interesting faction because they havent build a 40+ metric ton tank.
Back in early days of "Western Front Armies" the US Forces dont had a heavy Pershing tank (hey, how much Pershing tanks fought in ww2? Oh jeah. 20 Pershings...) and it worked (from my point of view).
With the P.26/40 you have a tank that could be balanced between Panzer IV and Panther. Okay. The P.26 dont serve with the RSI army but that is CoH, a competitive pvp game and not a hardcore ww2 simulation.
With the Semovente 90 you will have a super Marder, Semovente 105 can be used for AT and/or AI.
So there are weapons u can use without any problems.
Furthermore with the "limitation" of such a theater of war you can pull CoH away from those 1944~1945 scenario to a 1941~1944 timeframe. So u wont need heavy Pershings (and M3 Lee would be a nice cool new weapon).
So i cant see any reason why anyone is against any italian theater?