In real life, it's because "Christ" in Greek is "Χριστός" (Christos). It dates back to at least the 1700s, though there were alternate forms such as Xtemmas that date back to the 1500s and earlier. Allegations that it exists to take the Christ out of Christmas didn't start until the 1950s; the fact that even believers forgot where the X comes from does mean that there certainly are non-believers using it in that manner, but they are unknowingly still connecting it to Jesus Christ despite their intents.
Of course, in the relation to Kingsmas, then yes, it is solely an insidious plot to deny Space King, and such heretics deserve what they get.