I think there are three primary reasons.
First, including the whole of the Second Sino-Japanese War in World War II means it begins before 1939.
Second, in many respects, the Second Sino-Japanese War is mainly part of the broad sweep of conflict in China that followed the 1911 Revolution rather than part of World War II. It only got tied into World War II because Japan decided to attack the United Kingdom, the Netherlands, and the United States. Indeed, had Japan left alone the United States, they might well have not only succeeded in their attempt to secure the Southern Territories, but kept their conflict from being seen as part of World War II.
Third, there's not much film of the Second Sino-Japanese War.
There actually is a lot of fiction set in that period made by China as propaganda.
