Partially because the country was never homogeneous in ethnic composition. In addition, the moneyed class perpetually created division of wealth which has been exacerbated by the collapse of industry, globalization and of course, the financialization of the economy. So what you have is racial and ethnic division succeeded by class division, yet the former is all the unwashed perceive. The Deep State and it's adherents take this into account. You can not conquer what is not divided.