If I remember correctly, obtaining Shedinja in the main series was very...different. My guess is they weren't sure how to translate that into Go, and decided to stuff it into the research breakthrough box instead. Similar to how they made Spiritomb available only from the Halloween special research.
In the main series, Shedinja also has an interesting ability, Wonder Guard, which meant it only took damage from super effective attacks (fire, rock, dark, ghost, and flying), which offset the fact that it only has 1 hitpoint. Since abilities are unlikely to be implemented right now in Go, Shedinja will likely just be a Dex entry that dies in one hit to everything, rather than a novelty attacker/defender.
