Their slaves were freed by force of arms, but keeping this off the books was tantamount to saying, "We'd totally go back to enslaving blacks if the feds weren't on our case about it." Over time, morals have changed and, as people have noted in this thread, "slavery is wrong" is a universal truth as commonly accepted as "murder is wrong."
But the fact that it took them 148 years to ratify it...!? The shear hubris exhibited by their difficulty in uttering empty words to the effect of "our bad, we shouldn't have enslaved blacks" is astounding. How the hell can anyone claim that racism is gone when there's that much resistance to platitudes that affirm such a universally accepted truth? It's like someone having resistance to saying "murder is wrong," and then hearing people say he's no more likely to murder than anybody else.