There were slaves all through the north too...
In fact, Indiana was filled to the brim with anti-slavery Southerners who'd found it necessary to leave their homes in the Sunny South to avoid being persecuted for their beliefs. The Civil War could not have been won by the Union without Indiana in fact.
So, why is Indiana a "Red State"?
The answer is simple ~ they tended to vote correctly in this election. Illinois, on the other hand, voted incorrectly.