Oh boy, this really is a massive question to answer. Firstly, I will assume that your question is in regards to the witchcraft trials of Western Europe, from 1400's to the early 1700's. Secondly, due to the large scope of the question, i can only provide a somewhat limited response, based around historical perceptions of women, and the nature of their work during this time period. I must apologise as I have had a bit to drink, and so my argument may be lacking in a general flow.
One of the main reasons is essentially wound up in historical representations of women. A major influence was the the Biblical fable of Adam and Eve, where it was Eve who took the bite of the apple, and convinced Adam to take a bite also, at the urging of Satan. So it was woman who was so easily convinced by the Devil, and it was woman who lead to the downfall of man. It was generally held as fact by society during this time that women were more susceptible to Satan's temptations, and were often compared to felines (seen as promiscuous), eels (slippery and conniving) and having the rage of a wasp, as compared to mans lion like rage.
The Malleus Maleficarum (Hammer of Witches) an encyclopaedia of witches, was written in 1486 by two Dominican Inquisitors in Upper Germany, had much to say on the weaknesses of women, including their "predilictions" towards excessive gossip and subterfuge, as well as their tendency towards either great heights of virtue or great depths of vice, seen again as a reflection of Eve's act in taking the apple. Furthermore, the wrath of a woman is considered more intolerable then that of a lion or dragon, and all wickedness is little compared to that of a womans wickedness. A translation of the text gives us "What else is woman but a foe to freindship, an unescapable punishment, a necessary evil, a natural temptation, a desirable calamity, a domestic danger, a delectable detriment, an evil of nature, painted with fair colours."
Due to females being able to have multiple orgasms, they were seen as sexually voracious, and never satisfied. Men, meanwhile, influenced by Ancient Greek medicine of the humors, thought that semen, bone marrow and brain matter were all of the same, and so had to conserve it. As such women were likely to turn to demons and Satan to fulfil their voracious sexual appetite, the demons in turn being unable to create spawn without a human.
So as we can see, not only does the most holiest of books essentially begin with the fall of man through a womans act, the encyclopaedia of witches shows an open hate and disparagement of women as a gender as evil and manipulative. A lack of power by women, both physically and legally was seen to lead towards cahoots with the Devil to acquire power lacked, as their options were seen to be either to scold, curse, or place a spell on someone.
The role of women in society was also incredibly important. Whist men were held to be responsible for the more strenuous manual albour, women were delegated to looking after food, which without knowledge of moden day bacterial theory, would spoil and kill without warning. Children, incredibly susceptible to diseaase without modern vaccinations and healthcare, easily became sick and died young, which also occurred to the elderly. So the womans role was to look after the most susceptible areas to unexplained sickness and death, in an era of high superstition and limited scientific knowledge.
So not only were women seen as weak willed, spiteful and ill-made creatures, they were delegated to tasks with the least explainable losses. This is seen by modern academics for the basis in accusations of witchcraft were generally directed at women.
Sources
J Sprenger & H Karmer " Malleus Maleficarum, 1486" in Witchcraft in Europe 1100-1700: A Docummentary history of Europe.
M E Wiesner, Women and Gender in early Modern Europe
H P Broedel The Malleus maleficarum and the Constructon of Witchcraft: Theology and Popular Belief
J Knox The First last of the Trumpet Against the Monstrous Regiment of Women