This is going to ba a long answer..... Sorry.
It's not that the 10mm isn't as good a man stopper (with proper ammunition)
The 10mm was selected by the FBI for use in the field, their Firearms Training Unit "concluded that its recoil was excessive in terms of training for average agent/police officer competency of use and qualification," and the pistols that chambered it were too large for some small-handed individuals.
The FBI's tests revealed that a 180gr 10mm bullet, propelled between 900-1000 fps, achieved desired terminal performance without the heavy recoil associated with conventional 10mm ammunition (1300-1400 fps). The FBI contacted Smith & Wesson and requested it to design a handgun to FBI specifications, based on the existing large-frame S&W 4506 .45 ACP handgun, that would reliably function with the FBI's reduced velocity 10mm ammunition. During this collaboration with the FBI, S&W realized it could shorten the 10mm case enough to fit within its medium-frame 9mm handguns and load it with a 180gr JHP bullet to produce ballistic performance identical to the FBI's reduced velocity 10mm cartridge. S&W then teamed with Winchester to produce a new cartridge, the .40 S&W. It uses a small pistol primer whereas the 10mm cartridge uses a large pistol primer.
The .40S&W cartridge debuted in January 1990 but their pistols (4006) took a few months to hit the shelves. Glock in the meantime beat the S&W's to the shelves and the rest is history.
(But that's the topic of another question)
**To summarize, the 10mm reduced recoil load is an excellent man stopper but the same results can be had with the 40S&W round in a smaller and easier to manage pistol**
As far as law enforcement agencies go it makes sense to have one gun to train with. They couldn't do that with the 10mm. It's the same reason that most law enforcement agencies don't use the .45acp either.