The same reason any animal life migrates:
Slavery
the first Americans came from Asia and crossed what is now the bering strait it was land then. they came by foot to get food
yes.
The Americans migrated from Asia to the Americans in search of food and in search of a better climate.
The civil war they migrated because they where slaves for the south.
There is a theory that the North American Indians (so-called "Native Americans") crossed over from Asia by way of a land bridge that once existed in pre-historic times between the two continents - roughly along the lines of the Aleutian Island chain.
in what we call now New York State
Jim Crow was still very alive in the south and the North was about change and more equal opportunities.
The first "Americans" were able to migrate to the Western Hemisphere 20,000 or more years ago because North America was then connected by land to ASIA.
One thing that fueled African Americans to migrate was to be accepted as equal.
The south had became even harder to live so black went north for better treatment and higher ability to get jobs.
They migrate in the winter.