If I were a slave-owning farmer in the South prior to the civil war, wouldn't it have been in my best interest to prevent slavery from expanding, since it would allow me to produce crops much cheaper than any new competing farmers out west? Or was the line of thinking more about how it could benefit the country's economic output as a whole?
For economics it's important to remember the value not just of what the slaves in the antebellum south produced, but in the slaves themselves. They were seen as a commodity with a limited market, so there would be an obvious advantage to slave holders if they could expand that market.
The other motivating factor was political. A desire to keep close to parity the number of slave states in the union vs free states, and with that the number of pro slavery senators. There was a persistent fear among slave holders that the more populous north would be able to pass slavery restricting laws in Congress and that pro slavery control of the senate was needed to avoid either a slow death of slavery or outright abolition. In fact this attidude was one of the primary motivators for secession following the election of Abraham Lincoln: a man who campaigned and won with a platform of restricting slavery, in spite of not carrying a single slave state. He was the ultimate example of how pro slavery sentiment was now a minority within the U.S.