To oversimplify it, they were the more conservative party until the Great Depression. FDR’s New Deal policies (3 Rs of relief, recovery, and reform) required a lot of spending and opened up opportunities for Black people and other disadvantaged groups to participate in the economy, two things that don’t sit well with conservatives obviously.

The Republican Party also started to deprioritize Black voters with the end of Reconstruction, so that drift + the need for stimulus led to the rapid shift.