"Why didn’t Abraham Lincoln deport all blacks back to Africa; after all, they were definitely illegals, weren’t they?"

Actually Abraham Lincoln favored sending them back to Africa.   

"Lincoln first proclaimed an interest in colonization during his eulogy for Henry Clay in 1852, when he admitted his allegiance to the esteemed Kentuckian's dual creed of gradual emancipation coupled with colonization. If slavery could be eliminated and the slaves returned to "their long-lost fatherland," claimed Lincoln, "it will indeed be a glorious consummation."

"When Douglas tried to portray Lincoln as the friend of the blacks, Lincoln countered, as he did in a speech at Peoria, Illinois, by denying that he saw blacks as equals and by advocating the colonization of freed slaves in Liberia. Lincoln was aware, however, of the practical difficulties of such a program: "If they were all landed there [Liberia] in a day, they would all perish in the next ten days," he stated."