I think there are two factors:
The first is lack of practice. No one was born knowing how to parallel park. You have to do it in order to get any good at it.
The second is spacial awareness. There are again two factors here. One is awareness of the dimensions of your vehicle outside your lines of sight, the other is your ability to relate that to the dimensions of the parking space. Some folks just cannot visualize that relationship and will either err on the side of caution (stop six feet from the other car) or throw caution to the winds (park by ear or seismic evidence).
I suspect we've seen all the variations one might imagine.
Best
Les