There are several reasons, one of which is they lobby better than you do.
As a retired NY peace officer, I'll offer this: when I retired, my agency made me get rid of every pistol I owned with the exception of my duty weapon. They were able to hang this sword over my head for not complying: You require what is known as a "good guy" letter from your agency when you retire in order to get a pistol license. This says you're not crazy, weren't kicked out, and were in gun carrying status when you retired, thus eligible for a concealed carry license. No letter, no license.
So now you only have your service gun when you retire. Don't know if all agencies did this, but mine did. Officers were pissed that the one gun they allowed you, the magazines were now not compliant. By the way, in my agency, I had to buy all my own equipment, including weapon, magazines, carry ammo, etc.
While I realize many civilian shooters are highly trained, LEO's are documented in their training, have for the most part shown some restraint over their careers, have a working knowledge of the law, and are thought by some to be an asset to law enforcement even while retired.
So, not saying it's right, but there you have it. And to get back to my first comment, they know how to lobby, which is really what it's all about.