M1928 Thompson submachine gun in custom violin case
They were originally designed as a trench sweeper and a replacement of slow firing bolt-action rifles, of course it was between the wars and so it was never used as such. (Except for the mob, that is)
The gun got adopted due to the endorsement of some bigger branches of the military liking its quality, but it was always regarded as weapon which was too expensive.
The biggest issues with the M1 was that it was well over-designed, meaning production was slow and whilst it was reliable, it was an extremely expensive weapon to push enough models to arm enough hands.
Meanwhile the Sten, PPSH-41 and MP40 on the other hand where simplified but super effective designs that could be churned out way easier during wartime.
Ultimately when the M3 Grease Gun was approved later in the war that gun was significantly cheaper and easier to make. For example, a new M1 cost around $225 at the time… The grease gun cost $20 to produce.