the whole Manifest Destiny thing...you know when groups of white guys convinced they were god's messengers did a massive land grab from sea to sea, killing, burning, waging war on Native Americans, Blacks, and Mexicans... taking what wasn't theirs because they could...

That's usually the standard explanation. The north was an emerging industrial/capitalistic economy, and the slave states tended towards agrarian. Slave labor isn't all that desirable in factories and in merchant type economies. Europe also objected to it causing some diplomatic and trade problems with them.

Quite honestly there were many reasons. Some religious, some philosophical, and some were basically just raw power politics.

It provided the basis of wealth that put this country on the map as an economic power. If that industry failed the financial underpinnings that supported investment in other industry disappeared. The entire country depended on our primacy in producing cotton, and the reason we could produce it at such an advantage was free labor.

https://www.theatlantic.com/business/archive/2014/12/empire-of-cotton/383660/

By the time shots were fired on Fort Sumter in April 1861, cotton was the core ingredient of the world’s most important manufacturing industry. The manufacture of cotton yarn and cloth had grown into “the greatest industry that ever had or could by possibility have ever existed in any age or country,” according to the self-congratulatory but essentially accurate account of British cotton merchant John Benjamin Smith. By multiple measures—the sheer numbers employed, the value of output, profitability—the cotton empire had no parallel.
...
The industry that brought great wealth to European manufacturers and merchants, and bleak employment to hundreds of thousands of mill workers, had also catapulted the United States onto center stage of the world economy, building “the most successful agricultural industry in the States of America which has been ever contemplated or realized.” Cotton exports alone put the United States on the world economic map. On the eve of the Civil War, raw cotton constituted 61 percent of the value of all U.S. products shipped abroad. Before the beginnings of the cotton boom in the 1780s, North America had been a promising but marginal player in the global economy.
...
Slavery stood at the center of the most dynamic and far-reaching production complex in human history. Too often, we prefer to erase the realities of slavery, expropriation, and colonialism from the history of capitalism, craving a nobler, cleaner capitalism. Nineteenth-century observers, in contrast, were cognizant of cotton's role in reshaping the world. Herman Merivale, British colonial bureaucrat, noted that Manchester’s and Liverpool’s “opulence is as really owing to the toil and suffering of the negro, as if his hands had excavated their docks and fabricated their steam-engines.” Capital accumulation in peripheral commodity production, according to Merivale, was necessary for metropolitan economic expansion, and access to labor, if necessary by coercion, was a precondition for turning abundant lands into productive suppliers of raw materials.
...

Leaving aside moral considerations, which certainly had some influence.

Slavery meant plantation agriculture, and on a national scale, what benefits this tends to work against the interests of developing industry and of free-holding small farmers. A lot of the early sectional friction was over tariffs, which the north wanted high to foster domestic manufacturing, and the south wanted low to hold down expenses in procuring items a plantation economy could not produce locally. Bankers had a mixed view, as most plantations were mortgaged to the hilt, and foreclosure would cost money, but much capital was tied up in such mortgages, which produced less profit than mercantile and industrial investments, or investment in westward expansion of small farming.

The Mississippi River was the great artery for export of Midwestern produce, and products. An independent south controlling this could strangle at will the prosperity of the Midwest. It is worth noting that, while the duel in the narrow space between the capitols at Washington DC and Richmond is the focus of most casual interest and popular history of the Civil War, the main military effort of the Union actually focused on clearing the Mississippi, and it was success it this which sounded the death knell of the secessionists.

and needed slave labor to keep making money. If the Southern states were a separate country, it would put a kink in their money flow, and it costs a lot of money to keep Congress bought off, and to install a sympathetic President.
(Hasn't changed that much, has it?)

so the more states there were to support a free labor way of life, the better.

of the Civil War and one historian said that the war could have ended earlier if the people in the Northeast were more involved. Ivy League students were more involved with being on the college crew team than going to fight. This was a glimpse into the attitude of some of those living in that part of the country at the time.