It's simple
A gloss or enamel finish requires multiple coats of primer, undercoat, paint and varnish. All by skilled tradesmen if you want to avoid runs, blobs, drips and other imperfection.
Matt black can be slapped on by anyone holding a brush, in a single coat directly onto red lead primer, or even bare metal if in a hurry.
On top of that, standardising on one cheap, quick and easy product saved money, saved imports of dyes and pigments (quite likely from Germany) , and released chemical production capacity
All that green pigment would be needed by the army.....