The most likely reason(s) dante put sub-humans and beasts, such as charon and minos, in control of sinners in the afterlife is because
a) the catholic church taught it.
b) most people would find it terrifying if such subhuman beasts were in control of the fates of human souls.
c) his primary aim was to entertain his readers, who would have been familiar with these figures from greek mythology.
d) if beasts are in charge of assigning places in hell, then there is little point in trying to rationalize or bargain with them.