We’ve talked about this before on this page. it has everything to do with the difference in the way comics are sold.

Back in the days of what was then called Newsstand Distribution, your local candy store or whatever wouldn’t order titles specifically; rather, they would put in a general order for, say, 300 Marvel Comics, and that’s what the distributor would deliver. And these were consigned on a returnable basis–anything that didn’t sell was sent back to the distributor for credit. In that system, team-up books tended to do well, because nobody had to order them specifically, and an interesting team-up between two characters might make readers buy a particular issue in greater quantity.

These days, though, in what is know as the Direct Distribution system, every comic shop orders the specific titles and quantity of issues that they want to stock, based on what they believe they can sell. And they purchase these issues on a non-refundable basis, so anything they order they pay for, regardless of whether it turns over or not. This is what makes it so difficult to be a comic book retailer. As a result of that process, though, most retailers realize that customers tend to not want to pick up a team-up book with any regularity, either because they don’t like the characters involved every issue, or because of the feeling that the events in those stories “won’t matter”, or what-have-you. Consequently, team-up books have been a difficult proposition since Direct became the dominant distribution system.