Short answer to your question is: John Dewey. He changed the subject of history into that of social science. His reasoning for the switch was that the future of America would be industrial. If citizens were too educated they wouldn’t stand for working in the assembly lines as the nation would require. Therefore, not only was reading instruction delayed and made more complex (sight reading); social science was emphasized which downplayed the role of individuals and political liberty to social classes and economic struggle/causes. This change took place in the early 20’s/30’s and was given new life by the radical 60’s.
The long answer is covered in the book, America Revised: History Schoolbooks in the 20th Century by Frances FitzGerald. Here’s a review at the Underground Grammarian: http://www.sourcetext.com/grammarian/
P.S. One of the best books on Dewey, IMHO is
John Dewey and the Decline of American Education: How the Patron Saint of Schools Has Corrupted Teaching and Learning by Henry T. Edmondson III.