“Why do Christians accuse Wiccans of worshipping Satan and make fun of them, even though Wicca predates Christianity and Wicca is also a very loving and accepting religion?”
I can’t really speak for the why of a religion I do not belong to, although I can speculate, but I can correct at least one of your assumptions.
Wicca DOES NOT, by any stretch of the imagination, predate Christianity. Wicca is at best about 100 years old. Wicca is loosely based on older traditions, some of which *may* predate Christianity, or at least Christian influence in the areas of the world where those traditions were practiced. As a religious Practice, Wicca was developed in the first half of the 20th century.
We are loving and accepting of many things, but we generally do not recognize the “darker” aspects of life as inherently “evil”. Some of us work with trickster deities and/or deities often described as having horns. Both of these aspects may confuse someone who is entrenched in a duotheistic good vs. evil theology where the “evil” character is often depicted as having “horns”. Additionally, the Judeo-Chrisitian myth cycles include a legend of an Angel called Lucifer who “fell” and became the leader of the “bad” side. Early Italian Witches (not Wicca) often worked with a Roman deity known as Lucifer. Not the same guy, just the same name, but you know how this can confuse people.