You haven't been listening. William Wilberforce and those many English Christians who followed him did follow Paul's example. The Christians in the American North did follow Paul's example. It was the basis for their activism for abolition.
The "Christians" you really are talking about are the Christians in the American South, a minority of Christians, by the way. Many more fought for slavery's end than defended it pre-war. The reason teh Southeners defended slavery was because of the pride of place ("states rights") which is the sin of pride, and for economic and cultural reasons, which are the sin of idolatry.
Either way, the defense was sin, and we fought a war to be rid of it.
So why didn't 19th century Christians follow that example??: wish the slaves well, bid the masters regard their slaves as brothers, and remark that they could command if they wished it but wait instead for it to come from the heart???
Do not mix being an apostle with being a Christian. Every apostle was a Christian, no Christian since then has been an apostle. It was not our place to merely wish people well (that was answered by Jesus and James), but to do what is right before God. Abolition of lsavery was just one such right act. It was delayed by political, cultural, economic, and sinful reasons.