There are two main reasons that the United States supplied the Allied side and not the Central Powers in WWII.
First, the US was more closely aligned with the Allied powers. The US sided with them in part because most of the Allies were democracies like the US. They also were unhappy with Germany’s unlimited submarine warfare and the atrocities it was supposed to have committed in Belgium. For these reasons, the US liked the Allies better.
Second, it was much easier to supply the Allies than the Central Powers. While the Germans were trying to stop shipping to England with submarines, the British were stopping shipping to Germany. They were much more effective because they could use surface ships and could easily block access to Germany. So, it was much easier for the US to get supplies to England than to Germany.
For these two main reasons, the US supplied the Allies rather than the Central Powers.