Its a combination of A) lack of awareness of Italian war crimes and B) the fact that Italy got out early and helped the allies.

A) When most people talk about Japan and Germany's involvement in the Second World War, what do most people think of? The rape of Nanjing and the Holocaust. Now ask people to name a war crime that was committed by the Italians. Most couldn't. Sure some no doubt know about the brutal, genocidal campaigns that Italy waged to pacify its colonies. Or the forced deportation of Slavs in Italy's Yugoslav territories. These aren't at all "common knowledge". It also doesn't help that Italy's war crimes weren't investigated with nearly the same vigor that Japanese and German war crimes were. There was no Nuremberg trial for Rodolfo Graziani or any other Italian war criminal. Graziani was responsible for mass killings in Libya, Ethiopia and was one of the few generals to remain loyal to Mussolini, he was sentenced to four months in prison by a private Italian tribunal. These tribunals were overshadowed by the much larger Nuremberg trials.

B) It must also be considered that Italy did surrender early and switch sides. They contributed, albeit in a small capacity, to the defeat of the Nazis and Japanese. They were also a valuable ally in the Cold War. Both of these factors meant that the Americans and British were willing to "ease off", they were also more concerned with German and Japanese war criminals, who's atrocities were well documented and well known.