I suppose the only logical explanation of why the Italians decided to replicate the MkII rather than Mk3 scope would be that either they simply weren't aware of its existence, or they decided to standardize on the MkII since they perhaps had so many ex-UK (& Canada?) examples, either MkI, MkII or both? The MkI's could be upgraded to MkII quite easily, but that is also just a hypothesis based on logic. And logic does occasionally play a part in decisions of this kind!
AFAWK, the Mk3 so no service in WWII, or very little and AFAWK, the UKwas getting rid of MkI and perhaps MkII scopes postwar, not Mk3s. Of course that policy must have changed at some point as thousands seem to have got onto the market in as-new, matching condition.
Probably a case of one bureaucratic hand not knowing what the other was doing. Those new-in-the-crates rifles would certainly have come in handy in the 60s, 70s and 80s from what Peter has told us about the vast amounts of work required to keep the rifles then on hand serviceable.