I'm on mobile, but I'll try to explain nevertheless...

Due to Italy's entry into NATO (1949) and the beginning of the Cold War, the already timid efforts aimed at the de-Fascistification of the country's bureaucratic apparatus was brought to an halt and the allegations levied against most war criminals conveniently forgotten; few of them were actually tried and nothing like the Nuremberg/Tokyo trials took ever place over here, with the result that many things did not make the headlines or textbooks. These people's misdeeds have been deliberatedly hidden or forgotten for decades - but so were the German and Yugoslav war crimes against Italian citizens... probably as part of a perverted deal, see the Armadio della Vergogna - and until the mid-'60s our very own history programmes stopped at WW1. Italian historiography too used to be rather reticent on the matter and, in some cases, is still quite biased: every attempt at a retelling seems to degenerate into sterile political debates so there are not many good, comprehensive (or impartial) narratives out there.

Foreign historians do not necessarily hold a more balanced view; many don't seek access to primary sources and seem to do little more than repeating outdated notions or stereotypes. By just dismissing Italy as an incompetent ally whose actions mattered very little on the battlefield they're not just contributing to foster a negative - and mostly unjustified - opinion of our military but in doing so they're actually cementing the myth of Italians being "good folks"... what is thought of as a buffoonish Army is seen as clearly incapable of committing war crimes, and the dearth of Italian sources available doesn't facilitate the efforts to dispel those beliefs (which I believe to be closely linked).

Also - our country's relatively limited scope of operations, coupled with its perceived ineffectiveness and early fall do not lend themselves for a correct evaluation of the impact it had on occupied countries; most of the crimes our Army had committed (the 1924 - 31 'pacification' of Libya, the conquest of Ethiopia...) actually took place before the war and can often get overlooked in favour of more famous, or blatant, ones such as the Holocaust. I would like to conclude by saying that more often than not one can't have a civil discussion about Italy's role in WW2; both because of those long-lingering - not to mention defamating - stereotypes, but also because of many Italians' refusal (some of which is clearly politically motivated) to acknowledge their country's objective responsibilities.

These are, by and large, the reasons as to why postwar Italy was not held responsible for many of its government's actions... which remain to this day less known than those of its German or Japanese counterparts.

I would say, in short, because Italy was, unlike Germany and Japan, not uniformly anti-allied in sentiment or action. While the Allied powers were fighting against the fascist government of Italy, there was a real and active civil war going on as well.

The allied forces, in a lot of cases, were viewed as liberators. Mussolini was executed by Italians and not by the allies. I would say that Italy's case was somewhat more like Vichy France.