My guess is ease of manufacture. A simple textured tube seems like it might be easier to crank out than a hexagonal tube which I think would have to be milled. Of course I know little about manufacturing metal thingees, so maybe I am wrong.
12D's were HEAVY! Although, now that I think about it, it might have been an accessory tube to add length on to them rather than actually being issued that way. Of course one would think twice about dealing with a night watchman carrying a monster like that, which I believe was the whole point of them.
Def