IMO, the traditional distribution market began its slow collapse from the late 1960s through the late 1970s. During that time, the comic book only stores began to emerge (again, slowly, IMO). Phil Seuling and others went to DC and MARVEL with the idea of creating a non-returnable market, which, over time, became the focus for the publishers (for better or worse). It didn't mean the traditional markets completely disappeared, however; comics were still sold through bookstores (remember Waldenbooks and Borders, for example) and some other outlets, like Stop and Shop. 

For those people that shopped there, it at least put in front of them that comics still existed, and, if the pricing was right, those people may purchase an issue or two. Once the pricing got too high, though (like around $ 2.25, lets say), then those occasional shoppers began to drop off and those outlets stopped carrying the comics. 

 I have no scientific evidence to support any of this, of course; it's just my own opinion.