Because this is not what he has been sent to do here. He was not sent to be a politician. He was not sent to take the political side of each other. He was sent to the people. ALL people. Good and evil, slave and master alike. He was here to spread God's message of hope and love and kindness to ALL people. He wasn't here to throw it away. Slavery was an institution. It was there. It has been around since the beginning of time. As powerful people think they can control the weak. Participating in politics that he was not meant to take and discard one side over would have diminished God's message about him. He renounced certain choices and actions but never had people and what you and your kind meant by ‘abstaining from slavery’ to condemning and insulting people who had slaves. That was not God's purpose in sending Jesus.
If you study the teachings of Jesus, you will find that he never supported or talked about slavery. He does not take sides in this issue unless he uses it as an example and provides divine guidance for those affected by it. Because God knew that mankind would not, in those days or in the long run, give in to slavery. Criticism can only change the people of that day and many years to come, away from God. Knowing that slavery will exist in the years to come God has made laws that will make the institution as tolerable and humane as possible if His laws are followed until such time as human society has changed enough for people to give up.
But Jesus was not sent here to do anything but to correct the corrupt doctrine of God by the self-serving religious leaders, to establish the foundation of the non-Jewish church and to close the gap between man and God forever. He was sent to bring Gods Grace to ALL that he could join people's dramas or discard people's organizations. God Himself took / took care of condemnation and the issues of judgment.