Wicca is a specific religion and Christianity is a different specific religion. Wicca is a pagan religion and, of course, Christianity is not.
I think this problem stems from people having trouble leaving Christianity, probably having been brought up with this religion and feeling uncomfortable or guilty about leaving it.
Christianity is a polytheistic religion and Christianity in monotheistic
There are some Witches who practice some form of Christianity, but you have to realize that you don't have to be Wiccan to be a Witch. Witches may or may not be Wiccan.
Are you confused yet?
Yes, some people say you can be anything you want, but mixing pagan deities/gods with the Christian god can be a problem as the Christian god does not allow his followers to worship any other gods. I would advise you to think things over and do some research. There are some groups, of course, who say you can do this but to my way of thinking you dishonour both religions by doing so. And when you mix and match and include a bit of this and a bit of that you often end up with a big lot of nothing.