Because many old women associate more with cats and dogs have a longer history of being useful to men. To be a bit more clear, dogs have a longer and more useful history with humans whereas cats are not as domesticated (generally speaking) as dogs. Further, cats are more "self reliant" than dogs and are easier for outcast women to support.
Then too, we have the other dynamic of considering anything out of the "normal" as being "bad". This leads to people thinking that "outcast" women who like cats are evil or "witches". Of course, the same standard is not applied to men who are hermits...
Edit:
Seriously, do you want a full analysis of humankind's relationship with various animals as well as a complete analysis of how people view others that do not fit the "norm" in a Y!A limited format? There probably is some correlation between "eccentric" females and cats which leads to the assumption that "eccentric" females are "witches" and because there is a connection between "eccentric" females there is also a connection between "witches" and cats.
This is all balderdash, there is no real or historical connection between "witchcraft" and cats, and the only reasons that we think (or more appropriately believe) that there is is mostly traceable to Hollywood and relatively recent literature.
Superstitions about black cats made them out to be demon spies. Since witchcraft was deemed satanic, it was natural to link black cats to witches as familiars. All folklore and superstition.
Dogs are regarded about the same by ancient (and modern) Muslim clergies, especially the black ones, which they make out to be demons in disguise.
Mainly because the idea of "witch" came from the woman, living alone, saying she doesn't need a man, doesn't go to church, "knows things" about using local plants to cure ailments and....has a cat.
Surely, such a woman who isn't actively increasing the church's income by popping out a kid every 9 months, and instead of saying "pray to God to make your cough go away" provides you with some herbs and hot water must be a witch!
Back then, anything the church didn't like was called a witch.
Most people accused of witches were old women who lived alone.
Old women tend to collect cats and talk to them.
Nowadays we wouldn't consider it all that odd. Back then, it was looked on differently. Especially if the woman was anti-social or acted eccentrically--the way a lot of elderly people do.
If she was particularly adept at herbal medicine, she was even more suspect. Her cat--who was probably like a child to her--was considered by others to be a demon or "familiar" (animal connected to soccer.)
Black cats were considered extremely evil. Many a cat was burned or otherwise killed because of superstitions and fear.
But felines had their vengeance. Because of the lack of cats, there was nothing to prevent rats from spreading the Black Plague later.
But to this day, it's almost impossible to find a black cat with absolutely no white hairs--evidently, if any white hair was discovered on a black cat, it was considered a sign of "not completely evil" and the cat was spared.
Why were cats associated with witchcraft?
