I think another part of the problem with .40 is not unlike what happened to .41 Magnum, which is that 9mm is just about the most that an average, non-shooter can handle. Most police officers, especially in some area, are just about the oppose of gun people.
I'm not sure I completely agree with that, although there is some reality to part of that.
I think what happened was the increase in performance from the 9mm to the 40 S&W just wasn’t great enough to justify the switch, particularly when correct 9mm loadings are applied to the equation. It took about 30 years for the players to realize this but like all dogma, beliefs die slowly.
A while cops aren’t necessarily “gun people”, a 40 S&W isn’t a difficult cartridge to master. The recoil of a 40 S&W isn’t significantly more than a 125gr +P 9mm load. I’ve seen a few small statured people that struggled with pistols chambered for 40 S&W, but truth be told, they struggled with similar pistols chambered in 9mm. You could probably give those people a .22 and they would still struggle. Likewise, I’ve seen very small woman that could operate a 1911 in 45 ACP with extreme skill. It’s popular to attribute the LE switch back to 9mm from 40 S&W on poor shooters but the reality doesn’t bear that out. The reality is that the increased performance of the 40 S&W just isn’t that significant in the real world.
Yes, the 40 S&W has a bullet with a larger diameter than a 9mm and yes that bullet is heavier, but the 9mm is no slouch. The American attitude that bigger is always better is very prevalent but not always true.
As for the 41 Magnum, a couple of factors hurt the acceptance of that cartridge. The first was the two loadings, one very appropriate for LE (210 grain bullet and about 900 fps) and another that got lumped into the assessment of the cartridge in general. The fact that the guns were built on the N-frames gave little incentive in terms of overall size and weight of the revolver and the nail in the coffin was the word “Magnum” in the cartridge name (Yes, there were politically correct morons back then too).
In any event, the gap between the 9mm Parabellum and the 45 ACP appears large, but it clearly wasn’t large enough to make room for a third big player in the sandbox. It’s not that the 40 S&W is bad (it clearly is not) it’s that the 40 S&W just isn’t as awesome as originally touted.