[Wallace T.]

In 1860, the public works needed to irrigate cotton production in Arizona and California were still two generations away. The technology needed to pump water out of the aquifers to irrigate cotton was also not available. Additionally, before the construction of the transcontinental railroads and the Panama Canal, the cost of shipping cotton from West Coast seaports to manufacturing facilities in the Northeast and Europe would make it less competitive than cotton from the Southeast.

Beyond agriculture, the Western states were more conducive to mining, ranching, and small scale farming. I don't think you can make a case that slavery would have worked in the West at that time.

[x]

Cotton was grown on a smaller scale by Indians in Arizona and missions in California. Irrigation was indeed necessary. The Pima Indians had quite a sophisticated irrigation system for pre-Columbian America. When it was discovered in the 1880s that California’s climate was inhospitable to the boll weevil, California got more serious about cotton growing. While the big water projects didn’t come until the 20th century, I’m not going to discount the ingenuity of 19th century Americans when they put their mind to things.

The transcontinental railroads were already built by the 1880s. Romans and Spaniards used slave labor in mining. When the political will was there to create a slave society uses would be found for slaves.