Perception and, what was mentioned earlier, television conditioning. Shotguns aren't "cool" anymore. They're not slick. You can imagine yourself walking into a crowd of bad guys and leveling your automatic and mowing them down while your badge gleams on your jacket in the neon light. Then you get to say some smart ass comment and drop your magazine and slap in another one and go after more bad guys. Shotguns and revolvers are SOOOO spaghetti western country hick. Police, politicians, and the public are slaves to the media machine just like the rest of us. But in my observations, it seems that the old timers on the local force appreciate a good 870 more so than the younger members.
My wife and I go to our local range. We bring my Glock 23 and my Winchester 1300 Defender. She rents medium/large framed .357 revolvers because she likes their simplicity, weight, and the fact that she can use .38 rounds and not feel the recoil very much. We are currently trying to find one that "fits" her the best. We plunk rounds down range with the handguns just like everyone else, but when the Defender comes out, it gets the most attention....even from the local Marines from Camp Pendleton. Can't argue with the destructive power of 12 ga. 00 buckshot in skilled hands.