While it is true that the states that seceded from the Union were the slave holding southern state. Abolition of slavery while a growing issue was not the major factor at that time. Many people visioned the United States to be a "Confederation of Independent States"; not a political and physical union of one. They felt there was a right to secede from the Union or Confederation is there with her conception had been documented in the revolt of the original 13 colonies from the British Crown.
Abraham Lincoln and many of the Northerners on the other hand considered the United States to be a bound union of all states that which would be subversive to the federal government. The Southerners as noted felt that the states had rights which could allow them to leave the confederation of the other states. This concept is reflected in the name of the rebel states that the unit they call themselves the
The Emancipation proclamation was issue Jan. 1,1963 but it excluded the slave permitting States of Maryland, Delaware, Kentucky and Missouri that had remained with the Union. This started in 1862 as an olive branch extended to the Rebel States by exempting them from its clauses if they rejoined the Union.
Early in his political career Lincoln was not an abolitionist and in a campaign for Congress he did support the institution of slavery. Many Northerners although the states may or may not have had laws against slavery, supported the southern practice and indeed would return escaped slaves who were captured.