(*Modred starts to open his mouth but Lauderdale slaps a hand over it*) No, dear. I know you're just dying to tell us all the reason, but you're a fictional character. No one can trust your answer.
(*grins*) I think I go with the theory Eilis gave. We know that Rowling's wizards and wizards are not wizards and witches in the sense that Wiccans and other pagan groups are/were.
Plus, like she also said, and this is a much nicer answer in the context of the books, just as our world and its accompanying traditions and customs have changed over the centuries, the wizarding world would be developing right along with it. Not to mention we would have great influence over the wizarding world, what with young muggle-born magicians regularly entering it, along with many of their customs and beliefs.
(*sighs*) Not so great an influence in the reverse, unfortunately....