It isn't. X is the first letter of Christ in Greek.
But "I" is, I think, the first Greek letter in Greek version of the name Jesus.
Somehow, I often get the feeling that those who refer to Christians as "Xians" derive some sort of satisfaction at using an "X" to describe Christ.
I confess that I could be quite wrong about that. But I do wonder why they never refer to us as "Iians". My own guess is that "I" somehow does not have the same effect as an "X".