Remember the classic Cadillac ambulances? God, I always loved those. Coolest looking vehicles.
Also remember -- "back then", an ambulance was designed to quickly transport a patient to better medical care. Basically, medics would wrap a wound, put the patient in back, and take off.
Today, as mentioned previously, ambulances are practically rolling Intensive Care Units. A regular car chassis/frame simply isn't designed to carry all the weight of the equipment that modern ambulances carry, and also provide the room necessary for multiple EMS people working on a patient.
Okay, I've got to go look up some Cadillac ambulance pictures!!