I suspect because its use is cheap on two fronts. Firstly, the fire extinguisher itself doesn’t need to be a significantly structured pressure vessel, and secondly because the raw material itself is pretty cheap to make and fairly stable at room temperature.
The reason that CO2 comes out cold from an extinguisher is simply through its change in state from liquid to gas as soon as it leaves the vessel (Boyle’s Law).
On the other hand, Ammonium bicarbonate absorbs the heat of reaction directly from the fire itself to produce CO2, thereby reducing its intensity, whilst simultaneously starving it of oxygen also right on the fire itself.
Other than enclosed spaces such as cockpits, the ammonia would very quickly flash off and generally not be a major issue. The extinguisher of course should be directed at the fire and not at people, although that is not always the case.