It is because demodulation of an amplitude modulated (AM) signal is so simple that this type of modulation has existed for so long. The fabrication of a simple envelope detection circuit like that shown in this simulation is very simple and not costly.
Nonetheless, the rectification provided by a commutation diode produces some distortion, and this type of circuit is no longer used in high quality receivers.
Use the slider to alter the value of the time constant RC.
Click on and slide the rectangular lens on the curve at top right.