I don't imagine that constitutional amendments are fully implemented with immediate effect in areas that are congenitally hostile to them is why. It is not exactly news that the South did everything they could to resist the Fourteenth and Fifteenth amendments for over a century. They still failed - and why did they fail? Because those amendments were eventually used in a renewed federal effort to bring the South into the compliance with their most basic standards, an effort that was put on hold for decades. You can't expect history to happen overnight, sometimes it takes time.