I have two comments:
Firstly I know it might be slower to take more than one go to take a head off but it really builds anticipation for the crowd and especially for the next in line.
Secondly sometimes the traditional method has lasting effects that strike fear and education into the minds of the population and the elites. It would be better to educate the 1% in how many times the heads bounce in the town square when the basket is full than to have unbridled capitalism stealing from the workers.