The sympathetic nervous system is that part of the autonomous nervous system that is most often associated with the "fight or flight syndrome." The parasympathetic nervous system is that part of the autonomous nervous system that helps the body regulate homeostasis. It is most often associated with the body's need to "rest and digest" and "feed and breed."
Three reasons the sympathetic nervous system is not as discrete in it's activation as the parasympathetic nervous system are:
1.) The activation of a fight or flight response is not a gradual response to a perceived threat. The response requires action of some sort, and in a rapid delivery to the perceived threat.
2.) The activation is used as a survival adaptation, such as waking from sleeping to respond to a perceived threat.
3.) The activation is used to divert energy away from non-essential systems, such as the digestive system, and to essential systems used for heightened threat perception, such as the cardiovascular system.
The parts of the body regulated by the parasympathetic nervous system have more time to deliver the response needed, so the activation is more subtle.