I'll base my answer on the essay The Sino-Japanese War in History by Hans van de Ven, one of the leading Western historians on the Sino-Japanese conflict, and expand on the citations with a bit of a refutal of the point of view he describes. He begins:

By the 1970s, a consensus view had emerged in Western studies of the Sino-Japanese conflict. It held that the Nationalists had formed an incompetent, corrupt, and militarist regime that had been unable to mobilize Chinese society effectively against Japanese aggression. It had failed to make use of U.S. support to reform its armies [...] preferring instead to ready itself for a postwar conflict with the Chinese communists.

These accusations can be understood by taking into account Chiang Kai-shek's famous saying "Communism is a disease of the heart, the Japanese are but a disease of the skin" and his - either singleminded or clearsighted – refusal to let the Communists be Communists and ally with them against the Japanese, which he only did after the infamous Xi'an incident, where he insurgent officers imprisoned him until he agreed to ally with the Communists. I will not dive into the vast debate sorrounding this topic and just leave it at saying that the Americans, as can be seen in the citation, clearly thought it wrong.

As for the Nationalist Government being unable to mobilize Chinese society, that surely had not been for a lack of trying. There were incountable programs – for example Chiang Kai-shek's New Life Movement – started in order to drill Chinese society into a new shape, up to Dai Li's infamous Blue Shirts and other strings of Nationalist ideology often labeled pseudo-fascist. There had been several programs to jumpstart Chinese industry and economy, and extensive army modernization programs had taken place under German advisors (!) in the 30s; but the most heavily industrialised regions were Manchuria, already under Japanese control, and Shanghai, which fell almost immediately, and in the course of the battle took with it the 80.000 German-schooled elite troops.

The consensus about the Nationalists was most clearly articulated in Barbara Tuchman's 1970 Stillwell and the American Experience in China, in which General Joseph "Vinegar" Stilwell, commander of U.S. forces in the China theater and chief of staff to Chiang Kai-shek, figured as the quintessentially American hero - plain speaking, go-getting, dedicated, patriotic, honest, modern and rational. The Nationalists, on the other hand, were a politically debilitated "husk" who had wasted the United States' "supreme try" in China.

Stilwell was a very polarising figure, especially hated, both on a personal and professional level, by Chiang Kai-shek himself. His demand to be made supreme commander of the Chinese armed forces and other animosities led to several break-downs, and the actual feasibility, notwithstanding potential benefit, of many of his plans remains questionable. It is, again, a question to big to adequately talk about here, but I think it's fair to say that his single account of the war cannot be taken at face value when the personal views are taken into account. More on that later.

Tuchman built on criticism of the Nationalists by U.S. journalists such as Theodore White, of Henry Luce's Time magazine [...] In 1946, Theodore White and Annalee Jacoby's Thunder out of China became a best seller.

White had been a correspondent in China during the war and witnessed the Henan famine, in which approximately 4 million people died of hunger, first-hand. This obviously had quite an impact on him, and in fact he was relieved of his duty as war correspondent because the articles he sent back home portrayed the Nationalists to negatively (!). He later formulated his experiences in the said book.

It is true that the Henan famine was caused at least in part by the Nationalist government's decision to collect taxes not in cash, but in kind (mainly grain) in order to feed the army. Together with already depleted granaries, this led to the infamous famine. Again, I won't delve to deeply into this. In his book China's War with Japan, 1937-1945: The Struggle for Survival, Rana Mitter writes that "The Henan famine showed that the consequences of the financial squeeze on the Nationalists were something liek a Greek tragedy: individuals could have behaved differently, but overall the result was inescapable. [...] The continuing resistance of the Nationalist Government may have well beeen dependent on the grain tax. But those who paid the price were the peasants Theodore White saw dying in the field of Henan."

Characterizations of China as not a martial culture and one mired in corruption and backwardness contributed to dismissals of the Nationalists. [...] Stilwell referred to Chinese politics as "Byzantine" and spoke of "the suspicious, jealous Oriental mind."

And here we have good old racism. China was indeed perceived as just categorically different from the Western allies, or even from the Soviets – who, as you are quite right to say, have achieved quite a bit of attention even in popular media, though even that is a bit of a newer phenomenon. No comment on racism here, but I'll include another citation from Stilwell: "In 1942, he lamented that "the despised people [Chinese, Russians, Greeks and Filipinos] are doing the best work for civilization."" Make of that what you will.

Van de Wen closes his essay with this assessment:

The interpretations of the Sino-Japanese conflict outlines above have proved remarkably tenacious, and this has made it harder to have serious discussions about the Nationalists. Until recently, the weight of the field lay not with republican history, but with studies of Chinese Communism and late imperial China. From this perspective, World War II was of little consequence, as it did not have a significant impact on history; hence the paucity of studies about the Sino-Japanese War from a military point of view or analyses of its broader historical consequences.

This adds the ultimate piece of the tragedy: Even though the Chinese suffered immense losses, the actual capitulation of Japan itself was a result of the nuclear bombings of Hiroshima and Nagasaki and the Soviet invasion of Manchuria. The Japanese army in China only capitulated afterwards. There is of course, as you say, the question of the "Chinese quagmire": Without the Chinese resistance, how much more war material would the Japanese have been able to muster? If the Chinese had capitulated after the fall of Shanghai, or of Nanking, or of Wuhan, what would have happened?

But the Nationalist Government under Chiang Kai-shek decided to fight on, and survived, even though with immense casualties, even during the Japanese Operation Ichigo of 1944. We will never know what would have happened, and have to work with history as it happened.

That basically sums it up.

However, things seem to be changing. My university last semester offered a course on precisely the Sino-Japanese War for both Sinologists and historians, and relations with Chinese universities are pretty good.

In China itself, the Sino-Japanese War, and especially the role of the Nationalists, has also been recently reassessed. For example, read this passage out of Xi Jinping's adress at the commemmoration of the 70th anniversary of the war's end:

Today is a day that will forever be etched in the memory of people all over the world. Seventy years ago today, the Chinese people, having fought tenaciously for 14 years, won the great victory of their War of Resistance against Japanese Agression, marking the full victory of the World Anti-Fascist War. [...]

The new name of the wars themselves are also telling. The official stance is now that of the Chinese having taken part in a common effort together with all other free nations against the Fascist agressors - a good way to present yourself as a worthy partner on today's world stage.

[...] I pay high tribute to all the veterans, comrades, patriots and officers in China who took part in the war of Resistance and all the Chinese at home and abroad who contributed significantly to the victory of the War.

This is a remarkable change of heart. Compare this to the party line during the Maoist era, in which the Communists were the only ones who were actually fighting the Japanese, while the Nationalists were incapable at best and collaborating traitors at worst. Only in the 80s were Chinese historians allowed to write about the Nationalist contribution to the war again as part of a "new remembering".

Another important theme that has resurfaced during the "new remembering" is that of China's victimization during the war, namely the Nanjing massacres and the comfort women. This even extends to a "numbers game", in which Chinese losses against the Japanese are inflated to as much as 35 million people. The question of a conclusive Japanese apology and especially the controversy sorrounding the Yasukuni shrine remain hotly debated, and Japanese history politics under Shinzo Abe retain a nationalist and revisionist stance.

The Sino-Japanese War remains a topic underrepresented in the Western consciousness. I hope I could explain why that is and make a case for why it does not deserve to be that way. It really is a fascinating, though also deeply tragic and often just plain sad topic. I know alternative history is unprofessional, but it really is interesting to think about how China would look if the war had never happened.

Sources:

The Sino-Japanese War in History, Hans van de Ven

China's "New Remembering" of the Anti-Japanese War of Resistance, 1937-1945, Parks M. Coble

China's War with Japan, 1937-1945: The Struggle for Survival, Rana Mitter

Address at the Commemoration of the 70th Anniversary of the Victory of the Chinese People's War of Resistance Against the Japanese Aggression and the World Anti-Fascist War