Amos 3:3 “How can two walk together unless they are agreed?” Cultural homogeneity is the historical foundation for a successful society. Yes, if it is too monolithic, it can become stagnant yet unless the vast majority adhere to similar principles, the society will be torn apart by internal divisions. This goes for organizations as well as civilizations. If the "fringe" element grows past 5-10%, then cohesion suffers. Yet some fringe is necessary to keep challenging and refreshing ideas.
No civilization in history has survived multiculturalism. The great Muslim scholar Ibn Khaldûn chronicled much the same in the preamble, The Muqaddimah, of his magnum opus of history. From his perspective of the 14th century, he saw that societies started out strong because they were cohesive and lean. Later, after rising to power, they became decadent and fat, their ideals and officials corrupted through plurality of their empire. Then, would rise up at the edge of the empire a lean, cohesive peoples who would take over the greater or break it - because the empire was too riven with internal divisions to fight back effectively.
America is not strong because it is multicultural. It has been strong specifically due to its homogeneity of thought in most areas. If you see why America has survived and become preeminent, you can see just how relatively homogenous it has been throughout its history. That is not to say we have not had disagreements but by and large, until recently, the vast majority of Americans agreed on the most core tenents of life. Take religion, whether baptist or Methodist or other, Americans tended to be overwhelmingly protestant Christians or at least culturally so.
Culture is largely proscribed by religion. There are simply few places of agreement between a Muslim, a Catholic, a Buddhist, a Jew and an Atheist. They do not see the world in the same way or see the same things as valuable. While Protestants differ from Catholics and squabble across multiple denominations, this is arguing over the number of angels that can dance on the head of a pin by comparison. Atheists will come and loudly bray that this is not so, but there is no escaping it. In every country we see. Compare countries largely run by a Muslim majority, an atheist majority, a Catholic one, an old line Protestant one (Europe). There is much homogeneity there within them and vast differences between those categories.
RELIGION > CULTURE > POLITICS > LAWS