Cotton prices and profits were rising in the 1850s. Cotton planters thought the boom would last forever. Elites in the slave states convinced themselves that slavery was a good thing, the foundation of civilization. They weren't going to take federal money to give up their slaves. Given their expectations, it didn't make economic sense.
Earlier though the opportunity was there but not the will.
IIRC all states were on board with Madison’s proposal during the NW Territories negotiation- except for SC which was dependent on (rice or indigo, I forget) plantations.
There’s lesson for us today in how the interests of the ‘elite’ prevented the South from taking long-term economic actions that would have benefitted all.