It's very simple. At the end of WWII, communism is kicking ass in France and Italy.

In the first post-war elections for the unicameral interim Constituent National Assembly in October 1945, the [French Communist Party] became the single largest party in France with 26.2% of the vote and 159 seats.

[The Italian Communist Party] was the largest communist party in the West, with peak support reaching 2.3 million members, in 1947.

Then the Americans get involved.

In the May 1947 crises (or exclusion crises), the Communists were excluded from government in Italy and France. The crises are commonly reckoned to be the start of the Cold War in Western Europe.

In France, conflicting policies of members of the governing Tripartisme coalition created tensions, and economic conditions were dire under the presidency of Paul Ramadier. The French Communist Party (PCF) had the support of one in every four voters, polling the largest percentage of votes of any party between 1946 and 1956. Ramadier received warnings from the US Ambassador Jefferson Caffery that the presence of Communists in the government would lead to the blocking of American aid, or perhaps worse. ("I told Ramadier," Caffery wrote in his diary, "no Communists in gov. or else.") Ramadier began looking for a pretext to purge them. As the great French strikewave of 1947 began, a rumor circulated among the ministers in Ramadier's party, the SFIO, that the Communists were plotting a coup for 1 May, and the military was secretly mobilized. The Communist ministers opposed Ramadier in a vote on wages policies, and, on 5 May 1947, he expelled them from the government. The following year, the US rewarded France with hundreds of millions of dollars in Marshall Plan aid. No evidence of coup plot was ever found, and it was confirmed that the PCF had initially opposed the April strikes. The Communist Party's absence from government in France lasted well beyond the fall of the Fourth Republic, and the effect of this absence upon the party system and the stability of government have prompted historians such as Maynard Williams to describe 5 May 1947 as 'the most important date in the history of the Fourth Republic'.

On 1 May, Italy was thrown into crisis by the murder of eleven leftist peasants (including four children) at an International Workers' Day parade in Palermo by Salvatore Giuliano and his gang. In the political chaos which ensued, the president engineered the expulsion of all left-wing ministers from the cabinet on 31 May. The PCI would not have a national position in government again for twenty years. De Gasperi did this under pressure from US Secretary of State George Marshall, who'd informed him that anti-communism was a pre-condition for receiving American aid, and Ambassador James C. Dunn who had directly asked de Gasperi to dissolve the parliament and remove the PCI.