Juxtaposition means placing two things side by side so as to highlight their differences. Writers use it for rhetorical effect. Writers juxtapose divergent elements frequently: wealth and poverty, beauty and ugliness, or darkness and light
Juxtaposition can have the effect of absurdity or humor, or create a link between elements and images that appear unrelated until they are paired. Writers can also reveal truths about a character through contrasting their traits with another, to achieve a foil.
In essence, the author is stating that the degree of diversity that exists in the United States is much greater than any other country in the world. Any time there is a lot of diversity, there is the potential for problems.
