Oil shale
It was discovered in the region in the 1970s. it spurred economic growth in the neighboring areas of Colorado. however major a discovery it was, the industry declined leading to copper, silver and gold being the most important mineral resource in the Rockies mountain.
The doctrine of popular sovereignty was used. That meant that people in those states could choose to vote to support slavery and since they were majority then slavery would be permitted. There were even states that were at first non-slave states but then things changed and they voted to introduce slavery into the state and people voted for this and supported this.
Transporting goods over long distances", since this was far easier and cheaper than transporting goods over land.
