Because everyone knows she's the fairest in the land:

You read that correctly: three Snow White movies. One is set in 19th century China, and the third is a mystery. I mean, I don't know what it's like, it's not an actual mystery. It's not the sort of story you can easily rewrite - I know, let's make it a romantic comedy and set it the fabulous world of modern magazine publishing! because it requires seven short, unattached men who follow the heroine around all the time.

Disney's version wasn't the first, by the way; there were two in 1916, including one featuring a popular actress of the day, Marguerite Clark.

Here's more. What Santa Claus has to do with this I have no idea. I think it's something they just stuck in the beginning because the medium was new, and the rules weren't established yet.