From a reader:

I may be out-geeking you here, but D.C. did do a storyline to answer that
question in its retro WWII series “All Star Squadron” in the 1980s.
Hitler had a magic spear that turned magic-powered superheroes evil when
they got into its range. That took out all the most powerful heroes,
including the Big S because he is vulnerable to magic. The range just
happened to cover Western Europe and Asia, leaving Superman, the Spectre,
Green Lantern, Wonder Woman and most of the other heroes powerful enough to
end the war single-handedly on the sidelines while Batman, Dr. Midnight and
other human-level heroes were free to act abroad. Captain Marvel, who was
not a D.C. hero in the original conflict but was subsequently acquired, was
also covered in the retro series.
It isn’t really any more stupid than anything else they came up with.