nobody mentioned it's silly semi-rimmed case? That "feature" takes away from the round,
That's a commonly held opinion, today. For the first half century or so, it didn't matter very much.
the semi-rim case is a holdover from the early days, but has become essentially like the human appendix. Serving no real purpose anymore, and a potential source of problems.
All of Browning's early case designs have that semi-rim. The .25, .32, and the .38. Because, at the time they were designed, it was not a certain thing that chambering on the case mouth would work reliably. The idea was new, and rather unproven at that time.
By the time Browning designed the .45, the rimless headspace on the case mouth concept had proven itself in the field, and so the .45ACP is rimless.
The .38 Super, while "designed" a couple decades later, has the semi rim because it was created using the .38ACP case, with no changes to the case, only to the powder load, increasing the pressure. At that time (1929) it would have made good sense to drop the semi rim, and use a true rimless case, BUT, what "everybody knows" today wasn't something everybody knew (and knew they wanted) back then.
While the .38 Super was never hugely popular in the US, it has been very popular south of the border in Central and South American countries, many of which have laws forbidding "military" calibers, so the 9mm and the .45 were out, leaving the .38 Super to dominate private ownership in those places.
Also note that, until the adoption of the 9mm by the US govt, the .38 Super was as, if not more popular in private sales.
Today, we are inundated with choices that we didn't have in the past, and its easy to forget that this was not always so. There were decades when the only US made 9mm was the Colt Government Model and in those guns, the 9mm was decidedly inferior in performance to the .38 Super. later the Colt had to compete with the S&W 39. The only double stack pistol in 9mm was the Browning Hi Power (P.35), and it wasn't made in the US.
Later S&W introduced their double stack model 59, but it wasn't until a decade or so after that when the "wondernine" explosion of designs took place, leading to where we are now, with lots and lots of choices that simply didn't exist in the past.
The main reasons the FBI didn't choose the .38 Super in the fallout from the 86 Miami massacre are two. The first one is politics/prestige. And the other is that the Super didn't fit in the 9mm frame guns and the .40S&W did.