They CAN rotate the mag well. It functions as a dust cover in jump operations, but you cannot fire it.
The reason whey went with a horizontal mag feed is to get more prone and lower profile to shoot at. Pretty effective IMHO. The Bren gun did most of the dirty work. VERY accurate and reliable full auto.
the Volks-MP.3008 did have a bottom feed mag, it is the German ver of the STEN more or less.