I think that the zine stories that were being recced at crack van were online - and what makes/made it odd is that when some stories were posted online, the authors themselves did not mention they had once appeared in zines. So if what you say is true - that the medium controls, then you would not mention the zines in the rec - just link to the online story. But I am not certain why the authors would not mention the zines when they first uploaded them. I wonder if there was an impression that zine fic was "old" fic so better not mention it? (pure speculation)

Of course many authors did mention their zine origins and the Pros Circuit Archive went to great effort to mention the zines. But stories posted to the Pros circuit archive were done through a moderator who controlled what info was included. When I went looking for zine fic on A03 we found that many authors were not mentioning the zine origins.

In the end though, I think the lack of zine mentions does play a role as to why zines and zine fic is (and remains) invisible.