Question

Why did they end the Chinese Exclusion Act

Answer 1

Probably as a sign of friendship towards the Chinese, and to strengthen its international image.

Step-by-step explanation:

It would be bad for its image to discriminate against its ally.

Answer 2

China became an official allied nation of the US (and other Allies) in (December) 1941, (even thought it was at war with Japan since 1937 so it was already de facto allied) and the Chinese Exclusion Act was passed two years later, probably as a sign of friendship towards the Chinese, and to strengthen its international image. (It would be bad for its image to discriminate against its ally).