Blue Orange
Gone to Texas
I suspect it had a LOT more to do with the fact that many of these stores were stuck with large excess inventories of things like Red Box, which you could find on aging product displays in many places for a number of years after its release. It was pretty plain to see, after the initial rush, that they'd all drastically overstocked. Certainly in the period from 1981 through the mid 80's you could find these Red Boxes ALL OVER THE PLACE, along with some other products that I presume were probably a sort of package deal you got with the whole display stand. Nor did all these stores ditch TSR products entirely, as I recall seeing many copies of things like OA, DSG, WSG, and various 2e books (those softcover brown and green supplements in particular) sitting in various corners of many stores for years. I was in Vermont all through the 80s, basically, so I guess maybe things were different elsewhere, but I'd say it was definitely still possible to buy D&D stuff in B&N or the Mall into the 90's there. In fact I remember picking up various other RPGs in the discount box at B&N. Space 1999 being the one that instantly springs to mind, but several others as well. Obviously it didn't sell super well, lol, but equally obviously buyers were still willing to try stocking a few RPG products. Now, I recall that I started playing M:tG when that came out, and I'd say by around that time you MOSTLY had to go to the FLGS to get both D&D and Magic cards in one place, though the Kaybee in the mall in South Burlington, VT still carried some TSR titles at that time, as well as M:tG.
Bit offtopic but given the location it may be a useful datapoint: Manhattan Barnes and Nobles and Borders were stocking RPG books into the 1990s. Used to be one of my favorite things to sit with a coffee and keep up with the edition changes to White Wolf and D&D. (Yes, I bought them afterwards!)