spoiler alert! also why do Adam and Eve have belly buttons in all the paintings of them?
It's the same kind of logic where we get the notion that fashionable top hats were inspired by the Monopoly piece.
It is a bit of pointed anti-Semitism. While the whole story portrays Jesus as having been a Jew, the hatred of Jews is a huge part of Christianity. Thus, portraying Jesus as 'Christian' highlights the obvious problem of Jesus being a Jew, and in so doing, reinforces the denigration of Jews and Judaism.