Racial segregation in big cities of America didn't just magically happen. It was a policy, a plan, that was thought up and enforced. It happened in cities all over our country.
Chicago, the city I live in, is one of the most segregated cities in the country. It didn't just happen. Local governments made sure it happened. Baltimore too had an active segregationist policy that was instituted and enforced starting in 1910.
So, when people ask, "Why," and "How," we must acknowledge that there is a clear history. People made it happen and worked to keep it that way.