40K has been consistently popular in the US since the early '90s, when I had time and money to buy and paint miniatures and my group was into it.
Today most of the game stores in my area are 40K and board game focused, with some Magic: the Gathering and then RPGs something of an afterthought, usually only the latest D&D titles and a smattering of other new stuff. Amazon and .pdf downloads have basically killed the RPG-focused game store of my youth.