Because by 1940, the Japanese invasion of the Dutch East Indies became imminent.
These colonies were rich in oil, so such an invasion would ruin the energy stability of Europe and the U.S.
The Japanese invasion of China started in 1937 and cost about 30 million lives; it was associated with use of chemical weapons, forced labor camps, medical experiments on prisoners, and many other acts. However, these actions did not get a response:
On 12 September 1937, the Chinese representative, Wellington Koo, appealed to the League of Nations for international intervention. Western countries were sympathetic to the Chinese in their struggle… However, the League was unable to provide any practical measures; on 4 October, it turned the case over to the Nine Power Treaty Conference. — Wikipedia
For example, in U.S. the policy of Non-interventionism was very popular. In a 1940 speech, Roosevelt argued,
"Some, indeed, still hold to the now somewhat obvious delusion that we … can safely permit the United States to become a lone island … in a world dominated by the philosophy of force."
It is important to know that Japan not only invaded China; they also invaded and occupied Indo-China, Malaya, and Philippines, gradually moving South. Japanese troop build ups in Hainan, Taiwan, and Haiphong were noted.
Japanese strategists also had confirmed plans to take control of Dutch East Indies. And they actually did it January 1942.
In September 1940 Japan was assured that Germany and Italy would respect Japanese interests in the Asia-Pacific region by signing the Tripartite Pact.
On 24 July 1941 Roosevelt requested that Japan withdraw all its forces from Indochina. Two days later the U.S. and the U.K. stopped selling oil, iron ore, and steel to Japan.
As we know, instead of withdrawal, the Japanese government began planning military actions against the Allies.
Summary. Opposing Japan was not very popular in the U.S. and Europe, but the emerging threat of losing control over oil/iron resources plus a chain of escalating events embroiled the Western Allies in opposing Japan.