I'm not sure, probably the difference in timing and inevitability. It took a matter of months to tie in the assassination of Franz Ferdinand with all of the Great Powers of Europe being at war thanks to all the treaty obligations and such while it took 4 years, a trade embargo and a surprise attack to fully tie the war in Europe and the War in the Pacific into one greater conflict.
Maybe it is just my Eurocentrism, but for the huge toll of the Sino-Japanese conflict, it only covered, admittedly a large region, of Asia, where as once Russia, France and Britain got involved in World War One, the Theater of War covered Europe, Africa and Asia.