I’ve spoken about this several times in the past, and not all that long ago the last time. But in short, it has everything to do with the switch-over from the classic Newsstand distribution model that was prominent through the middle 1980s to the comic store Direct Sales model.

In the former, outlets weren’t ordering specific titles, merely a shipment of “300 Comics” all of which were returnable for credit if they didn’t sell. So a team-up book would make its way to the stands, where the audience, which was much more casual readers, would buy the books that looked the most interesting to them–so a team-up book stood a good chance of intriguing enough customers on a regular basis. In the Direct Market, each store buys the specific number of copies of each specific issue that it desires and thinks it can sell, on a non-returnable basis–so if the store guessed wrong, they eat the cost of the copies. And they make those decisions based on early solicitations. In that model and for that audience, team-up titles seem inessential, and so have never been ordered especially strongly. And there are far fewer casual readers looking over the racks for something additional to buy.