Why do secessionists place so much emphasis on the growth of anti slavery public opinion in the north?

The slavery institution was very important for the southern agricultural plantation economy. The rise of public opinion against slavery had made it very unreliable as the northerners were assisting slaves to run away into Canada.
  various enactment such as the 1850 act were meant to protect the institution, but it failed to pressure the North,leading to increased need to secede.

Social darwinism refers to the the theory that people who succeed in our economic system will one day out compete those who fail. one day the rich will out succeed the poor so we'll that there will eventually be no more poor