Why might the male slaves have been shackled and not the females?

Answer 1

Male slaves were more likely to be shackled than female slaves undoubtedly because males, with on average greater upper body strength than females, were at least potentially more capable of inflicting more physical injury on those who were keeping them in a state of slavery than their female counterparts.

A certain degree of gender bias may have been at work, with the belief in many cultures that men were more inclined to resort to physical violence than women and, therefore, had more experience in resorting to violence. Also, the use of physical violence in the service of a ruler / of the government was, in almost all cultures, was limited exclusively to males.

Answer 2

In all likely hood males were shackled, as opposed to females, because they were physically larger and stronger then females. When shackled it is difficult to move, thus the male slaves could have been controlled better.