It's a long story, but it involves the post-WWII economic boom, White Flight from the inner cities to the suburbs and away from the Black people in the cities (along with Redlining to keep the Black people from following them), the construction of Eisenhower's Interstate Highway system in the 50s, and many other factors I'm sure I'm missing.
Not sure why you’re getting downvoted - this is a well researched phenomenon. The interstate system opened up access (ie, subsidized) land for single family residential. And yes, one factor why white people moved there was to get away from black people - and even enacted covenants to keep out black buyers and real estate agents practiced what was called “red lining”.
This was turbocharged by court losses saying that cities had to serve all of their citizens. Many white people were okay living in the same city with brown people as long as there were white neighborhoods with schools, pools, etc. only used by white people. Once black (or on the west coast, Mexican) people were legally allowed to use them, some cities shutdown city pools and many people moved to suburbs where the infrastructure was technically open to all residents but since things like the design & covenants were created to limit who lived there, it was de facto segregated.
