The key to the 870's domination of the police market is actual performance, versus other brands.
This is not a matter of anecdotal stories like:
"My 870 failed on the first magazine load right out of the box"
or "My "X" brand shotgun has thousands of rounds through it, and has never failed".
The "bottom line" is, starting in 1950 you put hundreds of thousands of your gun into the hands of law enforcement, and then check the failure rate.
The real world fact is, NO modern shotgun has a DOCUMENTED lower failure rate than the 870.
When 95% of people are using a specific gun, they have to know something.
Again, this doesn't mean Mossberg's or Winchester's are bad guns.
It just means that in the real world of police usage, they failed to stand up, TO POLICE USE.
There are millions of Mossberg's and Winchester's in the hands of sportsmen that work quite well.
You can pick your own reasons why they failed, or why they shouldn't have failed, but the fact stands: They DID fail, and 95% of law enforcement uses Remington 870's.
I suspect the reason why police guns fail more than civilian guns is, civilians OWN their guns. We have to pay for them, and we can't afford to treat a gun like it's toilet paper.
You may not take care of your gun, but neither do you run over it with the car, or use the barrel to pry open a steel door.
Most police guns are owned by NOBODY and EVERYBODY.
In most departments, the shotgun is something pulled off a rack at the start of shift, or it's something that's already stored in the trunk or lock rack in the patrol car.
It isn't something used very often, and if something is wrong with it, somebody ELSE is responsible for getting it fixed.
Whether it needs maintenance is somebody ELSE's problem.
You just get another one from the armorer.
Most department armorers will tell you that most of their "repairs" to the 870 is replacing broken sights, splintered stocks, bent or crushed barrels, and heavy rust or fouling.
These repairs are usually traceable to either neglect, or more often, deliberate abuse.
Police seem the think the shotgun makes a handy "tool" for battering doors down, using the barrel as a pry bar, or a club.
Ever examined any piece of "community" property or equipment?
Take a look at a wrench you own. It may be dirty, rusty, and battered, but it works.
Look at a wrench used in a shop full of people. In all likelihood it's in terrible shape, if it actually works at all.
Police firearms are no different.
There's a difference between not taking care of YOUR gun, and treating the department's gun like it's used toilet paper.