There are a number of convergent forces impacting Italy's weaker occupation and rapid rehabilitation after the war. Sorry for only getting to this now; but this is a very complex question with many nuances (for this reason you and any other reader should also feel free to ask follow up questions!)

First and foremost, it's important to keep in mind that Italy was the first Axis power to surrender. Although Eisenhower and the American command was firm with the Italian government and demanded an unconditional surrender, the State Department grappled with establishing a policy addressing the attractive prospect of using Italian resources and material, even leading Secretary of State Cordell Hull to 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 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 in order to fight its way up the peninsula against the occupying German forces.

There's also an argument to be made that, unlike the very americanized and integrated German-american population, the Italian-American and Catholic voting blocks in the United States had a complicated and not entirely unfavorable view of Italy, making a punitive postwar policy untenable. Indeed the United States supported Italy's appeals against the harsher aspects of the final peace treaty, eventually encouraging the whole thing to be disregarded once Italy became a member of NATO. But there was an even stronger force in that both the United States government and voting public was concerned with the growing strength of the authoritarian Soviet Bloc, while simultaneously extremely adverse to getting dragged into yet another European war. The occupation of Germany was as much an effort to line divisions and tanks along the inner German border as it was to pacify the nation which had, in the public mind, caused two great conflicts in the past fifty years. This was already a lot for the American voters to stomach, so an additional occupation in Italy to defend against the Soviets might have been too much. It was much easier to just encourage the Italians to re-arm themselves.

Plus, with no actual Soviet troops having ever entered Italy (save for an attempted Yugoslav occupation of Triest which was not strictly speaking Soviet and was nonetheless rapidly expelled) it was very easy for the United States to lump Italy along with France and Britain in the category of nations to help back on their feet and in a condition to at least partially fend for themselves should Russian tanks breach the Iron Curtain (even though the necessity of the urgency with which the United States prodded Wester Europe to coalesce is another topic).

Another, somewhat interesting aspect, of the American attitude towards Italy is the consequence of the willingness to keep the Fascist administration intact in the immediate aftermath of the surrender. As discussed above, Italian public and private resources were exploited for the war effort; in order to do this, a large proportion of fascist public officials and fascist-tied business leaders had to be kept in place and indeed became 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 puppet government spiraled into Civil War. This encouraged 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.

This unintended consequence of US policy led the State Department to scramble in order to undermine the radical leftist forces in Italy; but that's in great part a discussion for another time. Hopefully I've at the very least provided a clear and comprehensive reason why Italy's post-war terms were as lenient as they were.

If you'd like to read more, there's the old Italy and the Allies by Norman Kogan, published in 1956. A more recent book with a more holistic examination of post-war Europe is Cold War Europe: The Politics of a Contested Continent by Mark Gilbert, published in 2014. Paul Ginsborg's 2003 A History of Contemporary Italy: Society and Politics, 1943-1988 is another, Italy-specific, resource you might want to look at (however I'm not entirely sympathetic to Ginsborg's internal political analyses).