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It's not that it's better, but it's been the standard for a long time and it works. The newer striker fired guns like the M&P, CZ P10, P320, VP9 are just as good, if not better in certain aspects. But Glock was the first widely successful striker fired LE handgun and has been the standard for so long that it's entrenched itself in good and deep. A lot of logistics goes into switching guns.
Lets take my department for example. We'll use 3000 officers just as a round number ( we dont have quite that number because there isnt a whole lot of people signing up for testing after the last year).
First we would have to get all of the armorers and trainers trained on the new gun. Then you would have to buy roughly 3500 new guns, holsters, extra magazines, magazine pouches, spare parts, specific tools, etc...
Then you have to train the 3000 officers on the new gun. Do you bring them down on work time? Who covers for them? Do you bring them down on over time? How much is that going to cost? Hiw much ammo is going to need to be shot for guys to get proficient with their new gun? Everyone is going to have to pass a POST qualification prior to carrying the gun, so we are looking at even more time to train guys that fail the qualification.
Remember most cops are not gun people. Many cops I know only have 1 gun, their issued handgun. I know a couple who dont even take it home at night. Other guys might have another handgun or two but they are also Glocks because that's what they carry at work and that's what they've been trained on for the last 10, 15, 20, 25 years. I know of detectives who keep their gun locked in their desk at work. They take it out for the mandatory quarterly range sessions and annual qualification. Some of these guys wont have much of an issue with the new guns, some guys are going to require a good amount of range time before they can qualify on the gun.
Then you have to get rid of the old guns. That means while the officers are training at the range with the new gun, you'll need several of your trainers/armorers off the line to get the old guns ready for trade in. Whatever distributor you contract with isnt going to want a Barrel 'O' Glocks showing up.
So with all that in mind, convincing management and city/county/state officials that going through all of that to replace a well known and respected firearm is going to take some serious effort.