Advances in ergonomics, higher strength and lower weight materials, better engineering and design optimization ... the new guns almost cannot fail to be easier to hit with accurately and quickly than older, more cave-man simple (sten, M3) or elegant but heavy and complex (Thompson) designs. Heck, even an Uzi feels a bit like a dinosaur relic in the hand compared to more developed, modern designs.
And that's leaving out the most beneficial aspect which is optics. A sub gun set up to (cleanly, efficiently, and ergonomically) use a red-dot or other low-magnification optic is head and shoulders above what passed for sights on most of these weapons.
Again, using the Uzi as an example, the sights are bothersome enough to find and get down on for a single aimed shot. Fire off a short burst and you're trying to find that bit of pop-can-with-a-hole-in-it rear sight and forget the front sight. Sure, many desgins can be retrofitted with some degree of success, but any "tactical" firearm that easily mounts an optic in just the right place (or allows you to put it exactly where YOU can see through it naturally) is just in a different league.
Though the heyday of the sub gun may have passed, the new ones are just better users. Progress marches on.