Why do some people think witchcraft is Satanic?

Personally I don't think it is satanic at all. Whether people are Catholic or practicing witchcraft, as long as they are no harm to others, it is okay. Some people probably don't understand or don't even try to understand. Sometimes when people don't understand something, they tend to quickly label it as bad or something like that. If you like to practice it, definitely keep doing so. Not everyone will understand, no matter how many times you may try to explain it to them.

Historically, Christianity has persecuted other religions, and they started to associate pagan gods with the devil and forced people to convert to Christianity. However, in the very beginning it was the other way around, where Christians were harshly persecuted against. Satanism and paganism/wicca are different religions.

Every now and again, religious cultures are gripped by what is known as The Satanic Panic. The most recent Panic was in the 1970s and 80's. It was embarrassing; people were seeing cults and covens everywhere when there were none. People were getting thrown in jail for illegal charges and even judges were caught up in it.

The core of the alleged satanic connection to witchcraft is the biblical verse "Thou shalt not suffer a witch to live." - Exodus 22:28. From that single biblical verse, the Inquisition murdered thousands. From the boogeyman story of Sodom and Gomorrah, all Abrahamism has given itself license to persecute people in the blind fear of being judged for their neighbor's sins for fear of being destroyed by fire from the sky. All over fairy tales.

It is shameful to intelligent thought.

Learn from the religions of the people who are giving you grief. That insight will help you learn how to combat the ignorance of people who do not understand you.