I think because after the WWII and during the Cold War, American "national security" political discourse was based on the stability of the international community. Therefore, the State developed an ideology or discourse in which political or economic instability around the world was perceived as a threat to its "freedoms" (grounded on the notions of individualism, democracy, freedom of enterprise -Capitalism). That explains the interventionist approach of US politics and expansion of militarism after WWII.
So, "American Freedom" was reshaped into an internationalist agenda focused on the expansion of its ideals and institutions. On the other hand, that ideology was not necessarily new, it was grounded in the 19th century, but the developments of the WWII as well as the antagonistic relationship with the USSR during the Cold War, gave the opportunity to develop a view of "American Freedom" that was interwoven with the stability of the international community.