The tommies can handle either the 30round stick clip or 50round drum. In almost any picture of WW2 Allied soldiers brandishing them, they have the stick. The Russians had great success with their ppSh, almost always with their 70round drum magazine (which they copied from the Finnish Suomi, also a round drum). So, why would Americans, British, and other Allies who pack the tommie forgo an extra 20 rounds? Is it simply a matter of the extra weight? 3 drums of 50 could carry as many bullets as 5 sticks of 30 - I would think they’d take up the same space. Was it some obscure military ‘we order it this way just because we say so’ kind of thing? Or is there some practical reason why Ordnance wouldn’t want an extra 20 bullets before reload?
From the wiki:
couldn’t be clearer, thx.
Drum magazines are notoriously unreliable. I didn’t look later for further details but I saw word that during the Aurora shooting his drum magazine jammed. I also saw reports that the ppSh magazine had jamming problems. Bottom line no military uses a drum magazine for good reason.
There is a classic photo of Churchill with a Tommy fitted with a drum, stogie in his mouth, looking grim.
If I had better internet, and knew how to make links, I’d provide it. 
EDIT: http://ww2.wwarii.com/wwii-people/government/britain/winston_churchill-thompson-submachine-gun
I guess that wasn’t so hard.
As others have said, drum magazines are more more prone to failure, compared to regular box magazines. Consider that replacing a magazine takes only seconds, the difference of 30 rounds is fairly academic. Just put those 30 rounds in two more magazines. These aren’t revolvers where you have to load one round at a time (at least, not until all of the magazines are empty, and you need to fill them)
The British were considering whether to drop the drum as early as April 1941. Reasons given in the files I have been researching in the public archives here were:
(a) Mechanically delicate
(b) Heavy and unwieldy
© Difficult to judge when drum is nearly empty
Note that the PPS43 used box magazines.
Grim? Looking rather pleased with himself I think.
For Churchill, those were both the same thing, yes?
Supposedly the Germans made many copies of that photo and attempted to use them as propaganda, painting him as a mobster.
They gave up on that pretty quick when they realized it was having quite the opposite effect from what they were aiming for, making Churchill look instead like a gun-toting cigar-chomping badass 
Never even pondered it just assumed it was drum.
Another site with some interesting thompson info is: Tommy Gun, Thompson Submachine Gun, Tommy Gun Prices, Tommy Gun Serial Numbers, Tommy Gun Value, Thompson Submachines Gun Serial Numbers, for Sale, .45 caliber
Not a single lecture on “magazine” vs. “clip”?
Shameful
Sighting down Naha ridge, I squeezed off the last 2 of my eight rounds at the bunker and was rewarded with the satisfying ping of the empty clip ejecting from my rifle…
Proper use for the descriptive era and mentioned rifle?
I’ve always thought that ad to be cool.
The OP has been answered, but here is a little engineering info on magazines:
The job of a magazine is to deliver cartridges into the correct position for the bolt or slide of the firearm to chamber them.
In manually operated arms (bolt, lever, slide, etc) there is normally an appreciable fraction of a second from the time the bolt is opened, until it closes. This is not the case with automatic and semi-automatic arms. The cartridge must move from below or beside the bolt (top feed has also been used, but is rare) to in front of it in a few milliseconds. In what follows, I will assume the magazine fits into the bottom of the arm, and feeds upward…other directions are comparable.
This motion is accomplished by a spring acting from below on the column of cartridges in the box. When there are only a few rounds in a straight stack, this is simple to accomplish, and seldom causes trouble unless the magazine is damaged dirty, or worn.
When high capacity is desired, things start getting more problematic. If the feed is to be kept simple and powered only by the spring in the magazine, then the spring must accelerate all the rounds in order that the top one is raised to the correct position, and this must be accomplished within a few milliseconds. The increasing mass as capacity is increased means a stronger spring is needed.
In addition to more mass, more cartridges will have more friction, and if they are in a double stack, (so the high capacity magazine won’t be unreasonably long) then the spring and inertial loads tend to force them outward against the walls of the magazine with as much , or even more force as is applied upward. This requires a yet stronger spring, which gives even more friction.
Eventually a point of diminishing returns is reached, and it is not possible to further increase the capacity of a box magazine further…typically this happens at around 30rds, give or take depending on a number of factors. Higher capacity box magazines are sometimes seen, but as a rule of thumb reliability decreases as capacity increases, and at some point reliability becomes so poor as to make an extreme capacity magazine useless.
Even if the magazine is able to raise a round into position in time, the top round may be pushed against the feed lips with such force, that the recoil spring is unable to overcome the friction to strip it from the magazine. Or the top round may bear on the closed bolt/slide with such force as to slow the cycling and reduce reliability.
A drum magazine eliminates most of the friction associated with the box design. Fingers within the magazine push individual rounds, or small groups of rounds, so that the force doesn’t all add up and appear at the last round ahead of a single follower. All the rounds still have to be accelerated to feed each shot, though, so there is still a practical limit to capacity, though it is often at or near 100rds. If the drum replaces a box magazine, then the final section often resembles a box, and some of the issues of the box design may come into play.
Very high capacity weapons use cartridge feeding arraignments considerably more complex than just a spring pushing all the rounds upward. Belt feeding is one option. A mechanical assist may be added: mechanical fingers lift the top round of a spring fed magazine into position, so the spring will have considerably more time to raise the rest of the stack. Many arraignments are possible, but none are as simple as a spring fed magazine, which is fairly reliable when kept to modest capacity.
More or less what my Dad said, who used one in WWII “The drum was too fiddly, hard to change and got in your way”.
I would think, with a straight magazine, a soldier would just store the full and empty clips in pockets or holsters on his chest or pants.
A bigger, round drum is a bit harder to store on your person.
LOL! I had this idea once for a SF movie where centuries in the future, some space marines are forced by an energy-weapon suppressing field to resort to 20th century technology. I envisioned this scene where one space marine picks up a Thompson and says “Hey, a Tommy gun! Like the cowboys of the old West used to use!” 