I suspect that it also had to do with the relative lack of light automatic weapons. In 1933, your choices were things like the M1918 BAR, or even heavier and more unwieldy belt-fed machine guns, or the Thompson, which was still both expensive and heavy. The Germans had a few good subguns, not that they would have been widely available in the US, and pretty much everything else came after the NFA, including all assault rifles.
Furthermore, the Thompson wouldn't have been all that well known as a military weapon. Other than some actions in Central and South America, subguns didn't see much military use until WWII. The average veteran had trained and fought with a bolt action rifle, and that probably shaped his views of what a military weapon should be like.