I thought the reason they wanted to secede was to allow them keep their slaves. No? Why would they want to keep their slaves so much so that they would go to wa

I thought the reason they wanted to secede was to allow them keep their slaves. No? Why would they want to keep their slaves so much so that they would go to war over that? I assume it was because slavery was good for THEIR (meaning the slave-owning class) economy. If as you say "Slave labor could not compete with free labor" then why didn't they (the slave-owning class) switch to free labor ASAP?