It would be interesting to see a formal study to determine if that is actually true... Relative to the racial make-up of the local neighborhood in which the church is located.

It seems to depend on both the geographical location, and the specific denomination of a church. Around me, I've see some that are a single shade, and others that are genuinely way mixed !

For one thing, the background of slavery. Caucasians and African Americans have a different take, a different interpretation of Christianity. People are tribal - want to be with their own cultural group. White Fundamentalists are often terribly racist.

Agree with @FortyTwo but would go even farther. This country, and many others, are becoming internally more and more homogenous/tribal. Whether it's religion, economics, politics, skin color or whatever we aren't challenged by different perspectives as much anymore. As @MsOliver pointed out there are a few cracks in this movement but not many.

Because they were so segregated by slavery

Yes, the social/togetherness aspect prevails here, and socially we're still quite segregated. And virtually none of the church attendees understand the chemistry of community.

This is scientifically known and understood. What's happening in their brains that gives them that 'feeling' they only get in church. The chemical response/reaction can be found/elicited in virtually any other communal activities as well.

Churches vary by denomination. Some denominations fit better for different cultures among races. Its also the difference in how one denomination views the beliefs of another denomination. Church is also big on community. It usually will reflect the demographics of those who live in the local area. I think just as in any community people will connect with those who are most similar to them.

Because churches are basically social clubs and the majority of people socialize with people who are like them.