First and foremost, it's important to keep in mind that Italy was the first Axis power to surrender. This means that between 1943 and 1945 the official attitude towards Italy wasn't as fleshed out as the practical necessities of war. Indeed, although Eisenhower and the American government were firm with the Italian government and chiefs of staff in demanding an unconditional surrender, the State Department had to grapple with formulating a policy addressing the attractive prospect of using Italian resources and material for the war effort, a policy which would even lead Secretary of State Cordell Hull to explicitly comment that it would be difficult to impose postwar sanctions on Italy if the present attitude continued. Of course, this didn't stop the country from being completely de-armed after the war, but the important thing is that between 1943 and 1945 Italy was more of a battleground than a belligerent, as German forces moved south to meet the Allies traveling up to occupy the peninsula. As early as the winter of 1943-1944, the State department had shifted its economic policy from economic relief to outright industrial rehabilitation as it collected all the resources it could to support the US army in its fight up the peninsula.

Another important consideration is the fact that because Italian public and private resources were exploited for the war effort, many public officials and fascist-tied business leaders were kept in place and could even become close collaborators of the Anglo-American forces. Parallel to this, the United States' military government ignored anti-fascist militias and revolutionary groups operating behind enemy lines as Mussolini's german-backed puppet government spiraled into Civil War, encouraging a deep divide between the internationally recognized but unpopular royalist government in the south and the antifascist militias in the north increasingly regressing into communist radicalism; two forces which should have instead been united against Mussolini's puppet regime. Perhaps hoping that the new postwar Constituent Assembly would legitimize those who had collaborated with the United States by granting them a clear majority, instead there emerged an otherwise unlikely antifascist-liberal-labor-communist block who very nearly gained half the seats and had no sympathy for the conservatives who had been left in power. A purge of fascists would, most probably, have strengthened and even legitimize this power block; something the allies were very averse to doing.

Of course, that's not to discount and discredit the importance and influence of the non-radical left in organizing antifascist militancy, it was their leaders, after all, who ended up governing postwar Italy. But the very fact that the Italian institutions had been left largely to their own devices is a testament to the fact that the United States' attitude towards Italy was disinterested in creating a wholly new political order as they thought it necessary to do in Germany. Indeed, the immediate post-war governance of Italy was nebulously split between the the allied command, royalist government, and antifascist militias governing autonomous communities, all in various overlapping spheres of influence. The situation was further confused by the fact that the Italian Communist Party and the Italian Socialist party were participants in the king's government, and although many of the militias were nominally affiliated to these parties, ultimate authority nonetheless was in the hands of the militia's leaders on the ground; causing no degree of unease among British and American authorities.

Under these circumstances; with formerly fascist-aligned capitalists selling goods to the allied occupiers and communist militants influential both in government and on the ground, the allied command adopted an all-told indifferent and hands-off approach with regards to Italian politics in the closing acts of the war, with only a markedly friendly disposition towards catholic authorities. Vatican secretary of state Mons. Montini and even tangential functionaries like Pietro Galeazzi frequently dialogued with members of the allied command; missives recount how the clergy was encouraged to give whatever support they could to members of the Christian Democratic party. A less open but nonetheless friendly attitude was extended to the monarchy, with both US state department and vatican missives corroborating the fact the American leadership was not averse to Prince Umberto succeeding King Victor Emmanuel after an inevitable abdication (the British perspective, although ultimately unheeded, was instead indifferent towards the monarchy and considered its role and fate it of small political consequence before comping out in support of a referendum on the monarchy's continued existence). All told, given this attitude it quickly became clear that any and all "justice" dispensed to the fascists would have to be administered by Italians themselves.

With Italian lawmakers left to their own devices, political discourse was more focused on broad debates on what kind of state the new Italy should be. The political mood became even more complicated over the course of 1945, when the antifascist unity which had characterized the final iterations of the Italian government as recognized by the allies rapidly unraveled, with the left's initiatives pushing for rapid local elections only headed off by fortuitous procedural doubts (the certainty of an imminent constituent assembly certainly also delegitimized the government's ability to rapidly enact policy, while the non-existence of a legislature predictably made legislating impossible).

I actually asked a similar question a number of years ago, and this answer from a now deleted user account might be a good start for you, although more of course can be added.