The simplest answer is that the Cold War was beginning to take shape by the time the U.S.-led Allied occupation of Japan had come to a close, with the Treaty of San Francisco signed on 8 September 1951 (went into effect 28 April 1952). The treaty once again made Japan an independent country and also effectively an ally of the U.S.

Ultimately, the outbreak of the Korean War on 25 June 1950 greatly increased Japan's geopolitical importance to the U.S. Not only did it give the U.S.-led UN troops a safe nearby logistical hub, but Japan was able to re-industrialize by selling military material to the UN troops for the war effort. Japan also became the "shield" of the U.S. against the Communist bloc.

Another factor was a general change in international relations, which also led post-WWII relations between the U.S. and Japan to be on an equal footing, at least nominally speaking. This shift rectified an issue Walter LaFeber cited in his book The Clash as one of the main reasons for the deterioration of U.S.-Japan relations prior to WWII, namely the western nations' refusal to respect Japan as an international power, even after its victory in the Russo-Japanese War.

Source: Walter LaFeber, The Clash: U.S.-Japanese Relations Throughout History