In the last several years there has actually been a huge increase in new scholarship and books about the Sino-Japanese War. We are much better informed now than previously.
As for the lack of knowledge beforehand, it boils down to a few main points.
A) Since no major Western forces were directly involved in the fighting, the China theatre has not been a popular subject for Western histories of the war which are dominated by their own countries' activities.
B) Westerners are very ignorant about Chinese geography, politics, and history. People knew who Rommel or Montgomery were. They have no idea who Li Zongren, Chen Cheng, or Peng Dehuai were. They have heard of Flanders and Paris and Italy and the Rhineland and can identify them on the map. They can't identify where Hunan or Sichuan or Wuhan are. This makes them less interested in what happened there, and makes it more difficult for them to understand.
C) The change from Wade-Giles to Pinyin made names of people and places even more confusing.
D) The victory of the PRC in the civil war made it very hard for historians to access war records of all sides to write a true history.
E) Cold War politics meant that people did not want to write a history of the war where China (now Communist) was more sympathetic than Japan (now an ally of the US).
F) The standard narrative of the Sino-Japanese War for the first several decades after World War II (that Chiang did no fighting, Mao did all of the fighting, that Mao's victory was due entirely due to popular support of the people, and that the Soviet Union did not help the Chinese Communists) was essentially a lie. Reporting the actual facts would make a lot of people who favored the "agrarian reformers" of Mao over the Nationalists look like incompetent idiots. This could not be allowed by the general leading liberal press because it would play in the hands of domestic conservative political opponents. "Who Lost China" was of such importance to US domestic politics that it became impossible to discuss the war without that influencing everything.
In the several years we have seen a huge increase in what's available, and I expect there to be a popular history book of the Sino-Japanese War became a bestseller in the next few years. The reasons are:
1) China is much more important nowadays. There is a popular interest in what happened there to better understand it now. This did not exist before.
2) The increase in China's importance has made more people aware of people and places there so they can have a map in their head of what happened while they read.
3) Internal US politics is unaffected by a true discussion of the war. People can now admit Mao did almost nothing to fight the Japanese, and that Chiang was much more willing to fight and effective than his detractors claimed. This doesn't impact domestic politics, nor are there people still living who would want to defend the claims they made in the 1930s and 1940s. Political opposition to the truth to defend their own reputations no longer exists.
4) There is increased awareness that much of the myth surrounding Mao and Chiang was just that, a myth. People are interested in what's really happened.