All retailers are in a bad way, as we, the buying public, have become accustomed to being able to get nearly bespoke versions of most of the things we want. Even things like phones, where we "buy off the shelf" get all sorts of custom features (our contact and messaging lists, etc.), and we fully expect to customize those after the fact.
Manufacturers are meeting those demands, too, offering all sorts of versions for their products.
Which creates a retail dilemma--there's almost no way you can stock every variation of every product simply. But, they can have a catalog front that can. The catalog front also takes up no floor space, and has no shipping tare to put "floor models" on display. So, items in the catalog are, by definition, cheaper than touchable stock on a sales floor.
Firearms are no different.
Brick-and-mortar LGS is barely going to be able to have more than 3-4 variant of a given arm from a given manufacturer. Which then multiplies with the number of manufacturers. And, naturally, if you leave one out, that's the exact thing a "walk in" actually wants to see. Which means your not having it a potential lost sale. Far easier for a web-based seller to get 5 of a kind of thing and get a discount for the "volume buy" and then let national marketing sell off the other four, than for a store to do the same.