Why Germany struggled with a 12.5 cm dual-purpose gun

Why did the Kriegsmarine have such difficulty developing and deploying a 12.5 cm dual-purpose gun?

It pursued an unfamiliar caliber and an idealized design when adapting an established 10.5 or 13 cm weapon would have been easier and faster.

The proposed explanation combines an unfamiliar calibre with excessively ambitious requirements. For the 125 mm weapon named in the question, the German armaments industry is contrasted with its existing experience of other gun sizes. Developing a suitable weapon in a less familiar calibre would add work to an already demanding dual-purpose project. An adapted 130 mm weapon or a long-barrelled 105 mm gun is suggested as a more practical option. The criticism is that pursuing an ideal design prevented acceptance of an adequate one.