It seems that there was a lot of open land with few white people (likely oppressors before and after slavery) on it in what we think of as the old west. To put my question another way, why did black people stay in the areas where law and society were not kind to them, i.e. areas with Jim Crow laws, areas with sundown laws, etc. ?
I think you may have to be more specific in your question here. Slavery was not abolished until 1865 therefore enslaved peoples could not have moved anywhere legally until this point.
Are you specifically asking about the post war era up to the 20th century? In the 20th century we see the first and second great migrations which, although later are similar to what you are asking.
If you could clarify your question I would be happy to go into more detail.