By 1945, the Axis Powers were Germany and Japan. While allied, their relationship was nothing like, say, that of the British and US. They were separated by nearly 9000km of distance (Berlin to Tokyo), different goals, and an inablilty to coordinate resulted.

The two were independent of each other. So when Germany surrendered on May 7th, 1945, there was still an entire other country the Allies needed to defeat in the Pacific.

World War II was never just about Europe. Hitler never led the entire Axis, his death merely took one of the nations out of play.