It doesn't even need to be an immigrant country. Although mass immigration is obviously a cause. A lot of countries have different ethnic groups because of past migration and conquest.

Even in countries that people assume are homogenous, different areas have a different language used historically (if not currently) and cultural or ethnic\racial differences.

In their rush to have contemporary diversity in their setting they fail to consider why such diversity exists. Not because it cannot, but because interesting world building and stories come with answering why it's there.