Why was the US so pro-China and so anti-Japan before WW2?

Japan was a threat to the US's Pacific territories, and those of other European powers. China was not.

Unlike China, and every other Asian nation at the time, Japan had defeated and humiliated a European power in the Russo-Japanese War. They defeated Russia not only on land, but also at sea where European powers typically dominate. Even if it was the ailing Russian empire, Europe considered the Japanese military a real threat.

Due to its modern and victorious navy and air force, Japan had the ability to project their power across the Pacific. They could threaten US Pacific possessions. In particular, the Philippines.

And Japan was increasingly militaristic and expansionist, whereas China had rarely ventured outside its borders.

Japanese militarism

Japan was very aggressive and tended to ignore and break international treaties. In 1895 Japan took Korea from China. This lead to the aforementioned clash with Russia in 1905 over their ambitions towards Korea and Manchuria. The European powers attempted to stave off a naval arms race and bind Japan with a series of naval treaties in the 20s and 30s which many in Japan found humiliating and eventually renounced.

The early Shōwa period saw Japan slide from a young democracy into an ultra-nationalist military dictatorship. This lead to concepts such as the Tōa Shin Chitsujo (New Order in East Asia) and later Dai Tōa Kyōeiken (Greater East Asia Co-Prosperity Sphere), the idea that Japan should rule over Southeast Asia; a direct threat to European powers' own imperialism.

To that end, Japan invaded Manchuria in 1931 on a pretense, ignoring international condemnation and withdrawing from the League of Nations. The conflict with China simmered for years until a full blown invasion in 1937.

World opinion turns against Japan

Japan relied heavily on trade with the US and Europe for oil, automotive equipment, steel, scrap iron, copper, and other resources. This in turned fueled their expansion. This profitable trade was left mostly unchecked, the US wishing to remain neutral. But the after the Japanese invasion of China in 1937 news of Japanese war crimes in China and an attack on a US Navy gunboat began to turn public opinion in the US against Japan.

Japan cozied up with Germany and Italy signing the Anti-Comintern Pact in 1936 and later the Tripartite Pact in 1940 identifying the US as their common enemy.

It was increasingly seen as hypocritical and self-defeating for the US to be fueling the Axis war machine.

In April 1939, Maxwell S. Stewart, economist and national chairman of the American Friends of the Chinese People, said before the US House Committee on Foreign Affairs about the Neutrality Act...

Our existing Neutrality Act was enacted with a view to protecting the United States against war. With that purpose I have profoundest sympathy, but I feel that in its present form it stands as a definite menace not only to world peace but to our own national security...

The time has now arrived when an immediate change must be made in the [Neutrality Act] if the United States is not to throw its influence permanently on the side of aggression. For, make no mistake about it, Hitler and Mussolini and the Japanese military leaders are counting heavily on the retention of the act in its present form. They know that as long as American resources are denied to their victims, while they, in turn, can obtain unlimited supplies of raw materials legally they need have no fear of continuing their policy of aggression.

Although this country has refrained from actually inpenalizing [sp] China by invoking the act, the weight of its influence has nevertheless been on the side of Japan. For although our sympathies as a people have unquestionably been on the side of China, we have contributed the economic resources without which Japan could not have carried out its illegal invasion of the Chinese Republic...

In all of this we are not only arming Japan against China, but we are actually arming her against ourselves. If Japan carries its aggressive program in the South Pacific farther and occupies American possessions, it can do so only on the strength of the sinews of war supplied by the United States.

Success for Japan in its illegal invasion of China would also endanger the security of the United States by encouraging potential aggressors the world over. Peace depends upon the general recognition of some standards of international law and equity. Each new victory for the aggressors undermines that protection and threatens to throw the world into complete anarchy.

From American Neutrality Policy page 263.

In 1940 the US passed the Export Control Act to limit sending war material to Japan, a likely enemy, and to also avoid shortages of those same materials in the US when war in Europe and the Pacific seemed increasingly likely.

But oil was excluded. The US and Japan knew an oil embargo could provoke a conflict and both were not ready yet.

Oil for the Japanese war machine

Japan needed oil for their industry and war machine. The US was a major exporter of oil at the time. If the US and Japan came to blows the obvious place for Japan to get oil was the Dutch East Indies (now Indonesia). A simple look at the map shows the concern for the US: the Philippines lies directly between Japan and their oil.

The US would be in a position to interfere with Japanese ambitions towards the Dutch East Indies. To secure their oil, Japan would have to neutralize the Philippines.

The last straw, seizure of French Indochina

After France fell in 1940, Japan got Vichy France to allow them to station troops in French Indochina which turned into an occupation of Northern French Indochina. In July 1941 the pretense was dropped and Japan occupied the whole of French Indochina positioning themselves to attack Malaya, Singapore, and the Dutch East Indies.

This was the last straw. As many in Japan expected, in response the US, Britain, and the Dutch cut off their oil trade with Japan. The Western Allies felt that Japan could not continue their wars without oil trade, and that the Allies could defend their territories. The Allies were right about the first part.

In December 1941, simultaneous with the invasion of Pearl Harbor, Japan invaded the Philippines. With the bulk of the US Pacific fleet sunk, the cavalry would not come to save them as had been planned. With the US fleet out of action, US forces in the Philippines on the defensive, and the British fighting for their lives in Europe, the Japanese were free to invade the Dutch East Indies 10 days later and begin seizing oil production.