Why do some people on the left criticize homogeneous societies while viewing opposition to diversity as racist?

I’m trying to understand the reasoning behind these two ideas rather than assume everyone on the political left thinks the same way.

Some people seem to view ethnic or cultural diversity within a country as inherently positive, while criticism of diversity can sometimes be interpreted as racism or xenophobia. At the same time, many countries historically developed distinct cultures, languages, traditions, and relatively homogeneous populations.

How do people who strongly support diversity view societies that want to preserve a distinct cultural or national identity?

Where do you draw the line between preserving a culture, preferring social cohesion, opposing immigration, and actual racial prejudice?

Interested in hearing different perspectives and the reasoning behind them.