Because the Gerät 341 was built in metal. I’m not sure if the 105 APHE shell was manufactured or just the specifications were known, but they were also corrected according to historical documents. The change increased its penetration but reduced explosive filler significantly.
The “fiction” of the Coelian is that a turret was never built and the guns technically don’t qualify as a unique component because they were built for the Kriegsmarine.
The fiction of the Tiger II 105 is that in reality it was a singular upgrade proposal which was rejected, while the tank in game combines three separate upgrade proposals (gun, engine, rangefinder) and has a physically impossible turret.
As you can tell, the Tiger II 105 is much worse than the Coelian in this respect.
The Coelian is basically in the same situation of the Tiger II Sla. The one component that was built (guns / engine) was meant for a different end user (Kriegsmarine / Jagdtiger). Installation was proposed as an addition (let’s mount the guns on a Panther / let’s mount the engine on a Tiger II). This installation was never completed because the component required to carry it out was never built (turret for the guns / engine deck for the Sla).
So, at the end of the day, the real answer was given by Smin earlier in the thread.
Developer discretion.