When and why did the modern literal 6-Day creation theory become popular?

Historically, Creationists tended to fall into two broad categories, those who subscribe to the Day-Age theory (the idea that the days in Genesis 1 represent ages of indeterminate length, not 24 hour periods) or the Gap theory (the idea that between Genesis 1:1 and 1:2 was an indeterminate gap, but the days were 24-hour periods). At least up until WWII, these were the predominant Creationists views, with the idea of a young earth being vehemently attacked even by some Fundamentalists and Creationists. > Advocates of [creation science] read the first chapters of Genesis in a way that allows for no life on earth before Eden and no death before the fall. > Until the last few decades most creationists would have regarded such notions as unnecessarily extreme. By the late nineteenth century even the most conservative Christian apologists readily conceded that the Bible allowed for an ancient earth and pre-Edenic life. With few exceptions, they accommodated the findings of historical geology either by interpreting the days of Genesis 1 to represent vast ages in the history of the earth or by separating a creation "in the beginning" from a much later Edenic creation in six literal days. > -- Introduction to *The Creationists*, 2nd Edition, 2006, by Ronald L Numbers, p.7 Jump to 2016, and the theory of a literal 6-day creation has become very popular, and is certainly the most talked-about of the creationist theories. In comparison, the Gap and Day-Age theories get much less attention, even being considered heresy (or nearly so) by some literal 6-Day creationists. What accounts for this change in attitude, and when did it occur?