Maybe because it is common? I mean the dual use as bunker, not the glory. I know that part of the main station of the U-Bahn in Bonn am Rhein, Germany has been built like that. Also several deeper stations built by the Hamburger Hochbahn in the 70ies are dual use like that. I guess for a certain timeframe (WW2 -> Cold War) it was like that everywhere where such infrastructure has been built?