The first reason would be socio-economic. With the confiscation of land by wealthy individuals during the conquest of the American west, many people remained landless and destitute; and many of them were European immigrants or African Americans. The invasion of unsettled unassigned lands was seen by them as the only way out of poverty.
The second reason would be economic. Many land speculators wanted to force the opening of these lands to make a profit for themselves or for the trusts they worked for (railroad corporations).
The third reason would be political. The Council of the Creek, which was dominated by the lower Creek who were favorable to the Confederacy had supported it and even provided men to fight for the South. This made their plight unsympathetic to the rest of Americans, especially considering that many of these Creek had owned African American Slaves.