Since people strongly believed in witches in the 17th century why do most no longer believe in witchcraft?

Mostly it's because as we learn more about our world we learn real explanations for the events of our lives. If we don't know why something happens, it would be easy to cling to superstitions. In the case of one example of the Salem Witch trials, a particularly wet winter resulted in a fungus growing on part of the grain crop. Many people who ate the affected grain started demonstrating symptoms of what local residents thought was a spell. Since they had did not understand microbiology and had no other explanation, when one of the local residents accused some of the local girls of being witches, everyone went along because they needed to find a cause so that they would be protected from having the same thing happen again. But we haven't removed all superstitions from our lives. Many people will still not walk under a ladder. Some still need to wear their lucky shirt when they go Bowling. Still others need to "touch wood" when they speak of some good fortune that has come their way, or blow on the dice when they're playing Craps in Vegas. Those are all examples of superstition and in reality statistics would be a better explanation for events.