Glocks are user-friendly. Cops forget to engage and disengage safeties. Keeping your finger off the trigger is much easier as shown by the fact that there are less AD's with Police who now use Glocks. Under combat, as proven by Police cases, it is just too complex for an average person to remember to disengage a safety to shoot, and also to engage the safety before holstering.
Agencies do not necessarily go with the cheapest. If they did, they would be going with S&W Sigmas, revolvers or many other guns.
Many agencies that can afford more expensive guns go with Glocks anyway, like Federal LE agencies who seem to have all the money they want. Even Spec-Ops guys that carry nothing but the best, carry Glocks. I know many that do, and they say that at least 1/3 of spec ops guys carry Glocks bought with their own money rather than the issue HK.
The fact is, Glocks just work, every time, no matter what the conditions are, they are simple to train, easy to shoot and go "bang" every time you pull the trigger, for years of constant shooting with little maintenence. What can I say, they're perfect, or at least as close as it gets
Ps- I have a couple 1911's, heavily customized, and my Glocks in my hands, in real combative shooting, are much more accurate and fast. The Glock trigger has the long take-up for safety, but once you are shooting, the trigger reset is so slight, that is shoots like a SA in rapid fire, if you know how to shoot a Glock properly, of course.