Because modern 9mm Luger JHPs are capable of meeting FBI specifications, and the ammo is cheaper, simple as that.
.40 S&W still has an energy advantage over 9mm Luger, but that can be overcome with overpressure (+P+) loads, albeit at the cost of most of the advantages of choosing 9mm Luger over .40 S&W in the first place.
Amusingly enough, many Law Enforcement agencies across the United States issue 9mm Luger +P+ loads, which eliminates many of the advantages of 9mm Luger, such as the cost of ammo, longer service life, and softer recoil impulse. In fact, just about the only advantage of 9mm +P+ over .40 S&W is a couple extra rounds in the magazine, which while nice, hardly makes up for the cost of re-equipping the entire force with different firearms chambered in another cartridge in the short term.
Furthermore, it's very likely that the very same scapegoating and knee-jerk reactions which resulted in the creation/adoption of the .40 S&W cartridge will be repeated in the event in which history repeats itself and an FBI-involved shooting goes awry. The only question is, will they go back to .40 S&W or drastically alter the parameters of their testing yet again, resulting in the creation and or adoption of a new cartridge?
Quote:
Originally Posted by The_Truth
10mm > 10mm short
P229 .40 was my first pistol. Absolutely loved that pistol. Traded it plus cash for a 1076 and never looked back. Now I own 10mm semi autos exclusively except my G43 which doesn't count 
I do still own a 4053, but I had BMCM convert it to 10mm for me  |
Just make sure that you're actually shooting full-power 10mm loads, otherwise you're just shooting .40 S&W out of a longer case and paying substantially more to do so.
I know that you probably already know this, (or at least I hope so) but you'd be surprised at how many folks brag about how they shoot 10mm not ".40 Short & Wimpy" only to embarrass themselves by posting a picture of their 10mm pistol sitting amongst boxes of ammo marked "10mm FBI" or otherwise with stats printed on them which show they're either equal or just marginally more powerful than .40 S&W.
Basically, if the ammo you're shooting is pushing a 180gr bullet at 1050fps, then that's 10mm FBI, which is essentially just .40 S&W in a longer case, sold at a substantially higher price. Full-power 10mm loads will push a 200gr bullet at 1200fps or a 180gr bullet at 1300fps.