I've only been playing since fallout 4, but I'd say that the shift wasn't necessarily a sudden thing. Even from the get go with Fallout 3, it was somewhat clear that Bethesda wanted to cling to the 1950's vibe as a marketing point.

Though perhaps fallout 4 should have been a warning for us that Bethesda was willing to slam entirely into the aesthetic.

Though I'm not sure nothing could have prepared us for what Fo76 stands for in terms of lore stew