The problem with selling GW games outside of hobby stores (GW or LGS) is the lack of support that turns someone from a kid buying a game in a box into a hobbyist, the painting lessons, the people to show you how the game works, that harms retention, the more people buying from somewhere where they're likely to get a good intro to "the Hobby" as a whole, the better.

X-Wing and boardgames don't have this hobby aspect and so they work well in other stores, with a GW (or other hobby wargame) you need paint, glue and more, while the newer GW boxes work ok without this as they have clip together models that not strictly need gluing and some, like Infinity or DZC give you a battlefield and terrain in the starter set, its still not a complete experience.