Why did the Lincoln make the war about slavery?

Lincoln in fact didn't make the war about slavery from 1861. On the contrary, he was willing to sign a constitutional amendment guaranteeing the continued existence of slavery where it was already happening. In 1861 his response was about armed rebellion against the government. It was the Confederacy that effectively made the war about slavery by refusing to accept the draft constitutional amendment as a compromise.

If you mean the Emancipation Proclamation of 1863: First of all, he needed to give the north a big moral/morale lift after two years where they thought they weren't getting very far. Making it a moral crusade meant the North would be much more willing to fight the long, costly war needed to overcome the Confederacy.

Second, it would make it much less likely the British or French would diplomatically support the south, which was needed as Britain in particular was being rather slack about enforcing trade embargoes - they had even sold 18 warships to the South.

Third, it raised the possibility the slaves in the south - note that the Emancipation Proclamation didn't apply to the slaves in Kentucky or Delaware which were in the North - might rise in rebellion. As it happens that didn't occur, although some slaves did actively claim their freedom e.g. Robert Smalls who sailed his boat out to the Union blockade. It also helpfully sorted out what to do with any slaves who were taken prisoner by the Union - they were just declared to be free and allowed to enlist.

But again, mostly it simply reiterated what had caused the crisis - the existence of slavery, the determination of wealthy southerners to preserve it against the determination of Northerners to get rid of it. Lincoln was opposed to slavery, getting rid of it was the whole reason why the Republican party was founded, and helpfully from his point of view the longer the war went on the more northern people came to dislike both the south in general and slavery in particular for causing it, and saw getting rid of slavery as the best reason for winning the war. So the Emancipation Proclamation actually followed public opinion on this.

Lincoln made the war about slavery as he wished for more black men to take part in the war as soldiers, some would argue that he had ulterior motives in the emancipation as he required more soldiers.