Because the butane or propane is in liquid form in the container, and vapourises at room temperature to give a constant pressure until it is exhausted. Using nitrogen or any other propellant which is not in a liquid phase in the container requires a *much* higher pressure in a full container to ensure enough pressure as the container empties, which means the container has to be stronger, so heavier, and more costly to make and transport, and the spray will be less consistent from full to empty. The safest and most environmentally friendly way to spray most household chemicals is by using a container mounted, finger powered, pump.
CFCs were ideal for the aerosol can application, being liquid at the pressures required at room temperature, didn't burn and gave controllable spray pressure by varying the mix in the container. Unfortunately, they act as a persistent catalyst which destroys ozone, and as a result are now banned.