@JelloYello Unfortunately this one is not ripe for serialization. The only ones that qualify are 2, 4, and 6. The reason is very simple: The one that got a serialization was the
only story in the first 3 volumes that
was not based off a syosetu (japanese wattpad) short story. All the other stories had authors that wrote the short stories, then they got adapated by an artist into a oneshot (artists, by the way, which have working contracts with ichijinsha, as evidenced that many of these same artists end up doing other serializations after the onesshots, which end up under the Zero Sum label).
The reason for this is very legal, and working with an author as well as an artist creates a lot of legal complications, vs working with just an artist who is already contracted with the company simply continuing their original story (which is what volume 2 story 1, and volume 4 stories 2, 4, and 6 are, original works by the artist). Now you may think "but hey, they adapt novels all the time!!" Big difference here is that novels that get adaptations are also with writers contracted to produce volumes with ichijinsha from their novel lines like Iris NEO into a manga line like Zero Sum. Again, it's all in the legal department. Syosetu is where mostly amateur authors get found as talents and eventually published, but the short stories the oneshots are based on are never actually published as short stories, they likely won a contest after being submitted and got a short contract that just allows Ichijinsha to use their story to adapt into a oneshot and nothing beyond that. To then turn it into an adaptation they would have to go back and consult the amateur author and negotiate a much more long lasting contract, one which the author may not even be willing to do, or may be a fussy writer unhappy without how the mangaka adapts their work (this happened with novel>manga Herscherik).
So, to avoid a big headache of further adapting these stories, only original works (non-syosetu story based) would ever be adapted further. Thank you for coming to my TED talk.