The Great Plains were not anywhere near fertile with the equipment available to the colonists. Very little water, ground that required very heavy plows and windmills, and a lack of fencing. not to mention a very long way from ocean ports critical to colonial economies.
Crops like cotton and tobacco also deplete soils pretty rapidly, and as a cash crop did so even more rapidly before modern fertilizers and seeds. Cotton was also limited geographically in where it could be grown in the U.S., as was well known by people well travelled in the 1840's and 1850's, which why some anti-slavery politicians didn't mind allowing slavery in the West or the North; they knew the 'Southern System' wasn't ever going to economically viable north of Mason -Dixon or west of a certain point in east Texas, and saw it as pointless to fight over re states outside the South. It wasn't even viable in the tier of slave states in the Union.