Because it offers the historian an objective snapshot of the public sentiment of the time, which the cartoonist (should be) distilling for their readers, according to their feelings, for mass appeal. I say objective as it is usually very easy to decipher their subjective viewpoint according to the publication. The value of this is that it is tapping into how the masses 'feel' rather than how subjective facts can be built to form historical opinion. It becomes especially valuable prior to this century, when public sentiment is harder to garner as we were less technologically advanced.
In the nineteenth century, Mexican American, Chinese, and white populations of the United States collided as white people moved farther west in search of land and riches. Neither Chinese immigrants nor Mexican Americans could withstand the assault on their rights by the tide of white settlers.
