The US has a huge German immigrant population, and during WWI many people sympathized with the Germans and felt they were basically forced into a preemptive strike because Britain and France would have declared war on them soon anyway. A lot of Americans wanted to join the war on the side of the Germans.

Then the Germans sank the Lusitania and became the chief bad guy.

In WW2, as others have mentioned, the US was more concerned with the plight of the British (would they hold out?), and then especially focused on Japan. Japan attacked Hawaii and subsequently the Philippines, which was owned by the US at the time and home to a lot of Americans.

Certainly the US played a role in every theater in WW2, but there were almost two different wars going on —- in Europe, primarily Britain and Russia versus Germany; and in the Pacific, primarily the US against Japan.