That's because Lincoln is an American hero. As an individual, he may not have felt ending slavery was as important as preserving the Union but he was on the side that ended slavery.
Lincoln didn't start that war but he was there when it was finished. The war was about slavery as far as the slaves were concerned and the war ended that "particular institution".
The south left the union because they could get a better price for their goods from Europe than the north was willing to pay. The north instituted taxes that would force the south to sell to the north. The south did not like the deal and wanted out of the union. Read Lincoln's first inaugural address to see how he really felt about freeing the slaves.
If the north was so hell bent on freeing the slaves, why was the underground railroad underground in the north? When the allies liberating Europe from Hitler came across concentration camps, did they set the people free or did they treat the people like the north treated runaway slaves? Honestly if the north was fighting to free the slaves, why weren't they free as soon as they were north?
Slavery could of ended with out a war. It's days were all ready numbered. The war was fought to prevent the south from becoming free.
Look at the killing field called Gettysburg, and you can see the future killing fields in Cambodia.