Answer:
According to the text, Lincoln seems to believe that if new territories were to become slave states, there would not be enough spare room for the surplus white population living in the Eastern states.
According to historians, Lincoln was not particularly interested in the morality of slavery, but he saw slavery as inconvenient. This is why he did not directly oppose slavery in the South, (only in the new western territories) until the emancipation proclamation, which he saw first as a military strategy.