At the end of WWII, the US's nuclear arsenal was still small and limited. The declaration of such a threat would have made it worth the risk for the USSR to dramatically ramp up their nuclear weapons research, which had been ongoing since 1942. The Soviets tested their first nuke in 1949; at that point or any time earlier, it would have been too late for the US to follow through. They would've had to drop the Marshall Plan and risk starting another "hot war". With their European allies, especially the UK, still struggling economically, the outcome would have been far from assured.