There's two main reasons. One is more pragmatic and the other is about long term political interests. The first one is that at the time, cotton exhausted the soil. Plantation crops, especially cotton couldn't keep being planted on the same soil economically. In this period there wasn't as much understanding of soil maintenance and health, and while there was some chemical fertilizer (Daniel Immerwahr's book, How To Hide An Empire gets into the importance to the development of American empire bird guano was b/c of its use as fertilizer) it wasn't as economical to transport into the south and use. So the Southern states constantly wanted new territory to expand to.

The political reason was that free states were expanding. B/c of the Constitutional preferences for slave owners in the Constitution, the South was able to impose their interests on the Northern states. But if the North population kept expanding faster than the south, and if the US added more free states, the South's advantage thanks to the 3/5ths clause would totally disappear and it's stranglehold on the Senate would be gone. At the time of the Civil War, the south had only about 1/3 of the population of the North. On top of that, 1/3 of the South's population was enslaved, so their interests were represented and they gave a representational boost to their enslavers against Northerners. So, if the balance of population kept shifting, the South, already weak in the House, would be totally ignored, and their abuse of minority power in the Senate would be totally sidelined. They had to keep expanding and adding more slave states or become a political non-entity.

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In addition to what's been mentioned as far as political power, it's also worth noting the role of capitalism and industrialization. As capitalism expanded into a global market and technology made it possible to process and produce things like textile products more quickly, you see the plantation system develop into a labor system, and white plantation owners relied on unfree labor to continue growing that. It's worth noting that even a lot of anti-slavery white Americans pre-emancipation are against enslaved labor not because they necessarily care about enslaved African Americans, but because they feel that it's a threat to free white labor.

Some sources to check out if you're interested in exploring the connection between capitalism and slavery:

Slave Country

Slavery's Capitalism

The Half Has Never Been Told