Young Earth Creationism by contrast was not very popular from 1900 to 1960 or so (even William Jennings Bryan was an old earth Creationist). That made a comeback in the 1960s starting when Henry Morris wrote "The Genesis Flood" in 1961, and that continued to pick up speed through the Moon landings (this incidentally undermines my earlier argument about Sputnik).

Part of that change could perhaps be attributed to the waning effectiveness of hiding behind 'old earth' as a way to keep on side with 'science'. Once the option of alliance with 'science' lost viability the natural approach is stake the in group identity as being opposed to any attempts whatsoever to conform historic beliefs to actual evidence. If you can't have an image of 'sane' then you go for an image of 'confident and uncompromising' - it is usually more attractive anyway.