Good question. It's because most of my flying is as an instructor - and then I only use the GPS for documentation - and occasionally for locating private strips or other user waypoints. I know the local area well and donot need GPS for navigation - and also do not wish to confuse the student with it. However, on private or business trips I mostly fly IFR and I always have the active route in the handheld, which I then keep up front.
My point was, that had I chosen a Garmin 296 or a NAV8OR, the size, battery issue and wires required would have stopped me from bringing it on every flight.