After the Civil War, many newly freed slaves migrated to the west because of the the Homestead Act that was passed in the year 1862. his act promised free land to all the settlers. 160 acres of free land or at a minor cost was promised to all those that settled in the west and had no criminal cases lodged against them for acting against the government of the United States.
I think that in delaying in mentioning the American colonies, Jefferson reveals his reluctance/lack of confidence in them and their fight for independence.
(*I didn't really know the context(I assume Thomas Jefferson), but just from a general character/human personality perspective, I believe this applies*).
Hope this helps! :)
