Cheap to produce, stamped metal, could churn them out by the thousands. Didn’t matter if they weren’t accurate, made for good suppression fire.
Pretty ingenious, too. On the M3A1, the wire butt stock doubled as a disassembly tool, using it as a wrench to turn the barrel nut, and tripled as magazine loader with a tab that was used to push down the top round of the magazine so another round could be pushed in, saving your thumbs.
Great video on the M3 and M3A1 by Ian of Forgotten Weapons:
https://www.youtube.com/watch?v=6ivr4QdhVtU