Why does Lincoln highlights these similarities and differences

The North experienced more immigration than the south. Southerners invested in slavery while Northerners invested in industry. The slavery issue increased tensions between the North and south.

Explanation:

By Frequently addressing, Northerners were real lovers of Lincoln. Opposite to slavery and its expansion were broadly upheld, so Lincoln's campaign promise not to increase slavery was affectionately greeted. There was a general understanding in the North that American way of politics had skewed too far towards protecting the attention of the southern states.

There was more migration in the North more than in the South. People in the North were engaged in manufacturing while people in the south were engaged in slave trade. Slavery created tension between North and South.

Lincoln wrote the following letter to Alexander Stephens addressing the differences.

Do the people of the South really entertain fears that a Republican administration world would, directly, or indirectly, interfere with their slaves, or with them, about their slaves? if they do, I wish to assure you, as once a friend, and still, I hope, not an enemy, that there is no cause for such fears.

The South would be in more danger in this respect, than it was in the days of Washington. I think, however this does not fit the case. You believe slavery is right and out to be restricted. That I suppose is difficult. It is definitely only substantial difference between us.