Because it fits Volks thematically and historically. The 6th man upgrade is pretty goofy for a late war faction that's supposed to be resource starved, including human resources (looking at OKW popcap), the best upgrades available to Volks that fit the game and are somewhat immersive are the StG and MP40 upgrades.
Im not a fan of the 6th man too, but as far as i know there was no lack of soldiers even 1945. Experienced and trained Soldiers for sure, but they had enough young people to throw at the front. And in coh2 Volks are a mix of inexperienced new soldiers and older ones. Anyway i think we should go away from this stigma (allies mass low qualitiy units, axis high quality low number units). We get nothing of value from this game design imo.
About volks: The mp40 upgrade is way more interesting than the stg one. Its not as brainless and gives the unit a clear design (mid-near range squad compared to ok on every range). And with it Volks are actually able to to beat upgraded Sections/Cons/Rifle if you use them perfectly. With Stg Volks there is no range to even contest them.