>>64057366
Because submachineguns were being made basically as expensively as rifles and machineguns during the interwar years.
Then WW2 happens, and everyone realizes "Oh fuck, we're gonna need shitloads more of these, and a lot faster!" thus virtually everyone hurries to develop more economized and simple versions.
This then ends up lingering for quite a long time, as it's easy to make a half-decent one which is dirt cheap.

Looking at Britain, they hurriedly have a captured example of a German MP28 reverse engineered to create the Lanchester. It works, but the British army just took it fully in the ass at Dunkirk and they're gonna have to replace a lot of equipment really quickly, and invest a lot more in SMGs than they had before. They can't buy more Thompsons from the Americans either, because they're too expensive and the U.S Government actually blocked them from buying more, the American Army needs them.

Thus they economize the everloving fuck out of the Lanchester, and you get the Sten, which is what they're looking for. The Sten isn't perfect, but it can be produced very quickly and cheaply, so that they can outfit British forces with enough SMGs in enough time.
Somewhat similar stories developed elsewhere in the world.