In part it was the "Anaconda Plan". but I think any Northrn government would have utilized their 1861 naval dominence to blockade the MS River. And there was no alternative system, by rail and river steamer, to make up for the loss of the MS River's exports. The Union could have chosen to not blockade eastern ports at all, and the Confederacy simply could not have moved enough cotton from the MS River Valley (the prime growing region) to these eastern ports to make up for the loss.
As it was, the increased price of transportation to England, due to internal transportation and the blockade, raised the price of cotton in england so high that alternate fabrics such as wool were substituted.