Civil rights started up in the 1950's. Jws were told not to get involved....after all the first century Christians did not try and change the laws regarding slavery. Jws could still preach and meet so they did not have to change it.
There is an article about this which I can't access at this moment.
Jesus told the slaves to be obedient to their masters so in effect supporting the idea and doctrine of slavery. Not a good thing to use as a back up to bad behavior