Warhammer 40k is, by far, the most popular build-and-paint war game in the US. Bar none. Other games have popped up that challenge 40k for dominance but they always wind up failing to maintain their popularity (i.e. Warmachine/Hordes, Infinity, Star Wars Legion, ASoIF, Mantic, Flames of War, Bolt Action, Malifaux, Frostgrave, etc.).
In May of 2020, Games Workshop operated 153 stores in the US, while it operated 140 stores in the whole of the UK. The UK is 40 times smaller than the US, yet it operated almost the same number of stores. The market penetration GW has in the UK is in a whole other league as compared to the US, where GW relies far more on independent retailers to market and stock their games.