In short as theorized by another commenter, with the M3 grease gun up and the military wanting to switch to it the Thompson, production lines were looked at for modification into other roles like an Automatic rifle or carbine as pictured here. I believe I remember the '06 gun beat itself to death and the the 30 carbine variant was seen as being far too heavy, complex and expensive compared to it's svelte and much easier to mass produce counterpart. However it's been a long time since I read that article so take all this with a grain of salt.