1 - Cthulhu lurks at the bottom of the ocean. Santa lives at the north pole. Sounds like a non-starter, right? WRONG! The north pole is in the middle of the arctic ocean, the closest land is somewhere in Siberia. While there is ice on the water there, it could hardly be expected to withstand the weight and heat from a massive toy production facility. No, if Santa lives on the north pole, he lives below the waves.
2 - Cthulhu has never been seen by mortal eyes. Neither has Santa. There you go.
3 - Cthulhu exists to corrupt the living, turning them slowly into fish-like zombies that he will one day devour. Have you looked at the average kid near xmas lately? How about mall people? Yep, Santa and Cthulhu are definitely on the same page here. Besides, Santa's pretty overweight for someone who lives in the harshest environment on earth.
4 - Santa is an anagram for Satan, another ancient evil who lies beneath our feet. Could those who first gave him this name have gotten the two confused? I mean, who can tell the work of one demon from another, really? And Claus, sounds kind of squamous doesn't it? Especially in it's proper pronunciation: Clow-sss. Perhaps Santa Claus means "squamous Satan"? Sounds like a good description to me.
So save yourself! Flee xmas while there's still time! BURN THE GIFT-BRINGERS!