developed with American-made 9mm ammo,
And this is also a big part of why the 7.62 Tokarev "fell out of favor".
WHERE is the "affordable" US made ammo??? And, I mean in the pre-ammo panic days. There was about none.
We got surplus East Bloc pistols, sold cheap, and surplus ammo sold even cheaper and Americans being who we are, they sold and were popular because they didn't cost much. But they never made it as popular cartridge, beyond that. And when the supplies of guns and ammo ran low and prices started going up, there simply was very little demand for the pistol(s) or its round on the US market at higher prices.
American ammo makers have a long history or loading European cartridges to lower levels than the Europeans do. This might have its origin in 19th century nationalism, but if one suggests that, its "tinfoil hat time"....
Regardless, its a fact that most "metric" rounds were loaded lighter than European specs until fairly recently, and some still are.
SO, Take a former Soviet pistol round, remove the low cost, don't have significant US made ammo in support, no new designs or guns made in that caliber, and you get what we have, essentially a milsurp niche round without many options or the commercial popularity creating a demand for them.
"I don't think GB adopted 9mm because that's what the Germans had, but because it was already a popular and effective round.
Great Britian in WWII is a unique case. First off, "popular" in terms of civilian use (and sales) simply doesn't apply. "Popular" with the military is the only real consideration. Britian went into WWII still firmly attached to the revolver and rimmed rounds. There was no "native" semi auto pistol or cartidge of any significance or in any meaningful numbers. (their .455 Webley self loading was not a big success)
SO, They turned to what they could get and one big one of those was the Canadian made Hi Power, in 9mm Luger. As Mike mentioned, they had been looking at the 9mm before the war, but the war turned interest into need. And when you can't make as much as you need on your own, you buy what you can get as you can get it. Canadian Hi Powers, needed 9mm ammo, and you CAN make enough of that...
The Sten was intentionally made in 9mm Luger BECAUSE it was what the Germans used. Intended for use by commandos, and others who could be outside regular resupply and could use captured ammunition. Numbers of them were (airdropped usually) sent to the French resistance, who could use them with Nazi ammo.
Later on the use of the Sten expanded to more regular troops and using captured ammo was no longer a high priority. After the war, they still had their 9mms, and ability to make ammo, so they just kept them replacing the revolver as main service arm along the way. When NATO chose the 9mm Luger as its standard, they were already there...and again, remember that all this was on the military side, there was no civilian side (or market) in Britain for semis or the 9mm. What little there was, was revolver oriented to the exclusion of just about everything else.
Which is, in some ways similar to the US and the 7.62 Tokarev, in that there was no civilian market or demand. US shooters got interested, for a while, because it was different, and it was cheap. When the cheap went away, the different was too different to sustain interest at the viable commercial level.
I expect the same thing to happen to the Makarov round, eventually, though it will take longer, because the Makarov can be fitted into existing popular guns, unlike the longer Tokarev round.