As I understand it when slavery was abolished in America "sharecropping"(a form of economic slavery) was introduced, so it's hardly surprising that many former slaves could not leave.
When was sharecropping abolished?
   It's an unpleasant fact that the slave trade made a lot of money for the merchant class in Britain. Just down the road from me is Liverpool, which has an impressive town centre full of huge neoclassical buildings funded by the slave trade.
   The majority of British people (the workers) at the time were also effectively economic slaves.

All the best PP