In part because of the authority of the uniform, but not entirely -- other authorities don't have nearly as much fetish appeal.
"Cops Tied Up" can be a revenge fantasy, giving the homophobes what they "deserve." Or a coming out fantasy: the big, macho, dominant cop discovers that he actually likes being dominated.
These aren't real cops, of course. They are BDSM bottom in the fetish uniforms available at any leather store -- that's why they say just a generic "police."
I've never met a real cop into BDSM, but I'm sure there are some. They probably don't use their real uniforms for scenes. A violation of professional ethics.
Many BDSM tops prefer their bottoms to be younger -- except when they're doing a cop fantasy. Then, the older, the better -- "two days until retirement, and the veteran cop finds himself in more trouble than he can handle."
If you want to be the top cop in a scene, you generally advertise by going to the bars in your fetish uniform. The tactic might backfire if potential partners think you're a real cop out to raid the place.
[Yes, gay bars are still raided, although usually the next day, after the damage is done, the police chief apologizes and says it was a "mistake."]
I'll post some nude cop pics on Tales of West Hollywood.