why was xena and her friend crufied?

Because Callisto's God wanted Xena to hell. So Callisto collaborated with Ceacar to kill Xena and Gabrielle.

If you pay attention to this entire series, this whole series has a religious underline in itself. If you want to call that one scene blasphemous you might as well call the whole series the same thing. They were crucified as a mockery to them. And jesus was not the only one ever crucified....just the most famous.