It's a Celtic thing. Actually...no. The main reason why witches are associated with the
Celtic fringe is that there were more witch trials in the hinterlands than in England. The
Scottish and
Irish courts, unlike the
English ones, allowed
torture to force confessions, and an accused witch was far more likely to be convicted there (through the vagaries of coerced
confession) than in England. The greatest number of witches tried, however, were in France and Germany, though there were witch trials throughout Europe.
It's a medieval thing, and reflects the power of the Catholic Church. Actually, the "Burning Times" were contemporary with the Protestant Reformation, when the Catholic Church was relatively weak. The Inquisition was more interested in Jews and heretics, not witches. Both Protestants and Catholics persecuted witches, just as they persecuted each other, and often, themselves, in the form of splinter groups. In an era where some towns (such as Amsterdam and Leyden) made large amounts of money harboring religious fugitives, no group of witches has been known to have applied for amnesty, or acted in ways that suggest an organized religious group, unlike Jews or Cathars.
It's a feminist thing, with a Mother Goddess, and all. Not according to historic witchcraft. Although there were woman healers, and midwives, some of whom were prosecuted as witches (although many more were prosecuted for knowingly selling poison as an accomplice to murder, which was also considered "witchcraft") there were male witches prosecuted as well, and there is no explicit mention of a Goddess in any surviving grimoires. Witchcraft was never, until modern times, considered a separate religion -- the crime of witchcraft was defined as apostacy (hence warlock -- oath-breaker, as in reneging on one's confirmation--as a term for witch), not paganism. To be prosecuted as a witch, the suspect must have been recorded as baptized.
It's a working-class thing: the witches were respected in their villages, and persecuted by the professional and ruling classes. Nope again. Mostly, "the authorities" in many cases, were loath to prosecute; many witch trials were at the hands of free-lance "witch hunters", who traveled from town to town and specialized in detection, interrogation, and eradication of witches, or the neighbors of the suspect her/himself. A witch, after all, was a dangerous creature, as anyone who has read more than a few fairy tales will attest -- which is why few of the villagers around her/him would try to drive the pest out themselves -- but there are cases on record where villagers petitioned many times to have a witch tried, and it was considered simply too trivial a matter to deal with. (This courtesy of a very good book, "The Witch and the Neighbors".) For what it's worth, Wicca, the modern faith, was originally supposed to reflect the beliefs associated with old aristocratic families, as in the H.P. Lovecraft story "The Rats in the Walls".
It's a traditional thing -- Wicca equals historic witchcraft. Sorry. The oldest "traditions" in Wicca have been dated with certainty only to about the late 1940's or so -- many of the phrases and ceremonies typical of witchcraft are taken from modern translations of Greek and Latin, with liberal interpolations from Aleister Crowley and some forms of Freemasonry. (Historic witchcraft, itself, was syncretic, and drew as heavily from contemporary Christian practises as it contributed to them -- yes, there are pre-Christian practises in the grimoires, but also please remember that Christianity in those parts in some places predated the arrival of some pagan groups.) My opinion is that even though one might claim that "even though it's not factual, it's still my religion, and you shouldn't criticize it", my own belief is that religion itself is weakened, not strengthened by false claims. If you wish to follow a set of beliefs that you think MIGHT have been espoused by the Celts living after Roman Britain and before the Celtic Church, it's better that you label them as a "reconstruction", or "supposition", and emphasize that their formulation is modern, and subject to revision.
But then, that's just my own belief.