It's a legal and social kinda thing, same reason why you hear "Happy Holidays" These days a lot more often than specific greetings for Hanukkah, Christmas, or Kwanzaa (or other I may not be as familiar with) because there are just a bunch of holidays around this time of year that a "Happy Holidays" is a general term for wishing them a good time for whatever their personal beliefs may be rather than trying to guess or being awkward about it.
Santa Gragas might be just a bit before Riot jumped on the "Holiday" bandwaggon, but at the same time I definitely have to say that Santa is a very iconic figure wherever you go. Whether some people may like him or not, chances are most will still know who he is and as such, who better than Gragas and his stout build to encompass the spirit of Santa, along with some body slams and probably an extra large serving of eggnog. Plus santa is a jolly figure head, he's meant to put some good humor on everyone's holiday cheer and make people smile. Happy elf skins probably were used because why not take the opportunity to throw in some bonus small-stature-race to fill in the Santa theme (plus bad santa veigar, though IMO he would have made a better not-so-happy-elf vaigar skin)
The snowdown/snowday/winter etc skins are there because not many holidays have Iconic figures like Christmas and Santa are, so they fill in with generic Winter/this-time-of-the-year themed skins for that wintery snow feeling (but hey, I live in florida. Snow is a myth around here ;:p)
Now, I'm sure Riot has thought of other religion-based skins along the way, though probably given a slight pat on the back when talking to whoever's in charge of their legal stuff with a "Well, it's a good concept, just wouldn't recommend it" due to specific religiously-iconic themes being a bit more dangerous since it might be easier to offend someone that way, probably the said someone's being people of those beliefs.
now, that aside.....WHERE'S MY COCA-COLA BEAR VOLI SKIN RITO. I KNOW YOU'RE IN GOD TERMS WITH THE COCA-COLA COMPANY. MAKE IT HAPPEN.
[slayer-jinx-catface]