Which is most responsible for the rapid settlement of the great plains?

 With the completion of the transcontinental railroad, and the recent indian skirmishes like 'The battles of Horseshoe Bend' and Jackson driving the Seminole out of Florida, and the Cherokee out of Georgian reserves with the addition of the Homestead Act of 1862, there was an extreme desire to populate the western frontiers that had already been somewhat settled by the Baptists/presbyterians/Methodists. All of these things Coalesce into the creation of an ideal situation where
A. you can receive land by simply caring for it.
B. You have the capability to have the goods you need shipped to you at low prices.
C. There are very few deterrents to the expansion with the Indian tribes spirits broken by the long journeys they had across the west, and their little knowledge with the lands they were put in.

Answer is mass marketing.