First, PC really just mean generally accepted by the masses (i.e. a political candidate is safe in using the word or floating the picture since most people are not opposed).
With this in mind, PC does not mean right, good, or otherwise, just means it’s safe to use in campaigning. At least as far as my understanding goes.
Your use of it only makes me think that you are asking why it is not widely accepted and unopposed.
My first consideration: No one knows what jesus looked like, we can only speculate based on what we conclude was his origin, and what the people there looked like… to the best of our ability. Many would even question his existence at all.
This leads me to think that most people that believe jesus walked the earth and is who he is claimed to be, are basically choosing which appearance they will consider true or preferable. It’s a choice is the point.
Santa is a fictitious representation of a person many consider a saint, but fictitious none the less. People have less issue with anyone altering fictitious characters, no matter how far they may deviate from their origin. Santa can be any skin tone, just like mickey mouse, and many won’t get to bent out of shape.
Jesus is believed by many to be a real historical person, and thus could have only one true appearance. And of course everyone who chooses their gods or deities also chooses what traits about those gods or deities they wish to consider true. So everyone has their chosen image of christ, many which differ from another, and just like any judgement or chosen belief, they defend it with all their might… hence, sure paint santa all day long, but Jesus, don’t touch.
Just my thoughts as they streamed to me. I’m not professing this as fact.