This is an important question for many reasons. First, America is a country that is mixed. In other words, it is not a homogeneous culture such as countries like Japan or South Korea. Hence, all the races must get alone or else there will be some great hardships. Race wars can truly destroy a nation.
Second, America is built on the ideals of self-evident truths, such as all men are created equal. So, racism would contradict the basic principle of the country. For America to be true to itself, there must be racial harmony.
The last reason why racism is an important theme is because America at times was racist in its approach to people groups. So, we constantly need to remind ourselves not to fall back into that place of darkness.