The better question is why not literally anything other than 40 cal. It's the most useless caliber of all the "popular" defense calibers. I mean, even if they were trying to standardize with literally any law enforcement agency, they'd use 9mm. Some agencies still use 45ACP because it's just what he people in their area know and prefer. But outside of Alaska, 9mm is pretty much the standard anymore (Alaska state troopers get issued 10mm since it serves as a dual purpose bear defense gun).

Literally no law enforcement agency since the 90s has used 40 S&W. That's why so many people on the civilian market use them, there's 10s of millions of police trade ins that have barely any use on them, making them halfway decent guns for dirt cheap. Or at least that's how it was until a few years ago.

But yeah, it's such a bizarre choice. The whole reason the government went to 9mm was because the limp wristed faggots and women who at this point were making up a decent part of law enforcement and federal agencies couldn't "handle the massive recoil" of anything else.

And apparently that's still true, since these buffoons have already shot each other in a few different incidents since they started training.