My theory is that the PC's are the exception to every rule so their opponents aren't aware that they can get up from unconscious. As far as their opponents are aware, someone who goes down, stays down. So it's not until they've experienced first-hand the PC's ability to 'miraculously' come back to life and continue to be a threat that opponents clue-in and start using coup de grace.
It's also my theory that animals don't care and will go for the kill shot out of instinct.
So what are your 'rules' for using coup de grace on your player's PC's?
In my game unconscious people don't just get up next round, as is common in a lot of D&D. Like in real life it takes time to regain your senses, to stabilize wounds and treat injuries that are serious enough to be life threatening, though a person can get knocked out and it not be life threatening, and then they can get up in the appropriate time period. I've been knocked out and up and in fair to middlin trim in about thirty seconds. But then again I've been injured badly and knocked out (which is a different kinda thing) and that you don't get up from in a minute or two. Knocked out is one thing. Knocked out and badly injured is an altogther nuther thing.
Non-human characters can sometimes recover much faster in my game though, more like ordinary D&D.
However, I think for the most part though the effective delivery of a killing stroke, and an opponent who is good at killing, will depend upon two things. Setting. What is common for that setting, culture, enemy, way of fighting, etc? And do folks really just jump up from near death, shake it off like condensation on a coke can, and go back to being dangerous? If so then you most especially need to assure that don't happen regularly, or you assure you don't live to see the grand-youngins. If the point of combat is to survive it, then you make that point by assuring the other fella don't. There is no middle ground with killing. If one or both parties are determined to kill, then you only have winners and losers. There is no win-win. But with lax effort and sloppy technique there can definitely be lose-lose.
Secondly, though probably more importantly, is the question of, "is the opponent a professional, or not?" A professional soldier for instance, or warrior/combatant, with a truly dangerous enemy in a life or death struggle leaves nothing to the imagination, or to luck, or to chance. He kills his opponent. He does not leave real threats unaddressed.
If I were in a fight to the death, I would kill and make sure I had killed. If my opponent were a professional, and knew what combat and killing was all about, I would expect him to attempt the same. Hope is not a plan, and killing is not a recreational sport. You either mean it and make damn sure of it, or you take the chance the other guy knows more about it than you do. And that's a real bad way to get an education from the other guy in the field. Better to do your homework on the subject and then finish what you start.