Most touring types would be very difficult or impossible to get out of in an in-flight emergency, and the seating arrangements would mean wearing either a seat- or back-pack would require some major surgery to the seats to make them fit.
That accounts for 80-90% of light power GA (probably). A small percantage of this category might have the option of strapping a parachute to the airframe.
The remainder - the ex-military or aerobatic kit - generally no real practical reason for not wearing a parachute, some folk choose not to - but that is invariably a reasoned conclusion arrived at after weighing all of the factors, rather than being in ignorance of the issues. Personally, I do wear a parachute if the option is available and practical.