What it boils down to is the original Robocop became a classic because everything worked, everything clicked into place.
Peter Weller was perfect as Murphy and Nancy Allen was great as Lewis, Ronnie Cox was good and slimy as Dick Jones and Kurtwood Smith was absolutely amazing as Clarence Boddicker. The supporting cast was just as good especially Miguel Ferrer and Dan O'Herlihy (boy did they screw up his character in the sequel).
The action was gritty, brutal and never boring, the dialogue had so many memorable quotes you might as well memorize the whole movie.
Most of all the story made us care about a man who had everything taken from him. His life, his wife and child, his body, most of his memories, his free will, and yet he still holds onto his humanity. He perseveres and overcomes every obstacle thrown at him and in the end manages to smile as if to say "You can do whatever you want to me but I will always be Alex Murphy."
You can't duplicate what made Robocop what it is just as you cannot duplicate what made Star Trek, The Thing, Psycho, Halloween, The Texas Chainsaw Massacre, Night of the Living Dead, etc, what they are. These movie aren't all perfect but to a degree they all caught lightning in a bottle and they endure and have become part of our culture!
Some remakes have been entertaining but none of them have captured on ounce of that "magic" thing that made the original so memorable.