What Farkel said, plus....
I suspect it became a liability concern/fear. I used to work the food stands at every DC, 20+ years. Toward the end, I saw for the first time concerned-looking and very serious "brothers" with hand-held thermometers running around between food stands, checking the temperatures of piles of sandwiches, frowning, and sometimes hauling them away.
I think they got scared that, while a JW sickened by contaminated food would never sue them, someone's "Bible study" or "unbelieving mate" would have no such qualms about bringing a lawsuit. I further suspect the lack of a restaurant license in such a case would not bode well for the Society.