You will notice that just about all 9mm Para submachine guns magazine are straight. There is probably a reason.....
Ahhh...but arguably the most successful (popular? functional?) 9mm sub-gun is the MP-5 and it uses magazines curved to follow the line created by 9mm cartridges nestled together snugly, with that ever-so-slight taper to each one. As did the very successful Sterling. (And also the Vz.61 Skorpion, though in .32.)
And that's the opposite direction from the angled magazines of a 1911 or Luger.
On the complete other hand, you can run rounds through a Glock or M&P or xD, etc., every bit as fast as some sub-guns with a much lower rate of expected feeding failures than most 9mm sub-guns tend to produce. I mean, if you asked which is more likely to run reliably at high rates, a Glock or a MAC-11/9 ... well, you wouldn't even have to ask.
So the question is a bit more of a head-scratcher than it would first appear.
I think a lot of it might have to do with basic simplicity. What's the easiest high-capacity box magazine to make? A square box with no tapers or bends or angles. A lot of sub guns are built around that idea of greatest simplicity. Of course, when you're putting the magazine into a separate mag well forward of the grip (Sten, Thompson, M3, S&W-76, Reising, Beretta Model 38 or 12, and on and on...) why not make it as simple as possible?
When you want the compactness and extra barrel length of the Uzi or MAC, then the mag goes into the grip, and especially back in the days before advanced polymer frames and grip assemblies, that means a bulky, more perpendicular grip shape.
