Gotta love revisionist history. I would wager that slavery wasn't even a top 3 concern during that time period. Sure, it probably was to the slaves, which is a no brainer. But to politicians, I'd say it probably wasn't. A good way to think of it is this. Nowadays, politicians don't even care about the poor of America. So if they don't care about actual voting citizens that are struggling financially now, do we really think they cared about a people that weren't even citizens and meant essentially nothing to their re-election efforts? I don't.
This isn't to say that slavery wasn't wrong. It most definitely was. However, as it has been said before, I believe Lincoln used slavery as divisive issue that would put the Union in a positive light and the Confederacy in an "evil" light. Much like our current POTUS has done with Dems and the GOP (The dude thinks he is Lincoln after all). I don't believe Pres Lincoln really gave a crap about blacks much like I don't believe Pres Obama really gives a crap about illegal immigrants. They both just use them as issues that they know the other side is defensive about and will come across as callous and cold about. To be clear, the difference in this is that at least Pres Lincoln was in the right, even if his motives weren't pure. Pres Obama isn't in the right, per se. There are many ways we can tackle immigration that are humane and fair. But that's another thread.