I'll play devil's advocate and simply point out that Disney/DVC has 16+ years worth of merchandise sales figures and DVC membership numbers to drive their manufacturing decisions.
I think the biggest issue may be the fact that members only see brief snapshots of what is available during any given trip. In the relatively short 5 1/2 years since our first DVC stay, I've seen quite a wide variety of items offered. I have seen MANY different shirt designs--t-shirts, polos, long sleeve, short sleeve, tank, men's designs, women's designs, resort-specific, generic, and so on.
I have also seen magnets, Christmas ornaments, glasses, keychains, jackets, bags, license plates, umbrellas, caps, fanny packs, water bottles, luggage tags, resort artwork, can cozys......the list seems endless.
To me it's more of an immediate gratification thing...just because they don't have a particular item when you visit doesn't mean they have never made such an item or that they never will again. If we want to continue seeing new products they have to phase-out the old.
The resorts we tend to spend the most time at are the BoardWalk and Saratoga. IMO, both seem to carry a good variety of DVC items. SSR's shop is fairly small, but about half of it is devoted to DVC items. If I could only visit one location, that would probably be it. BWV usually has a good selection along the far right-hand side of the store (if standing on the BW looking toward the store).