For the races you haven't played as, why haven't you picked them yet? A personal preference against that race or just haven't gotten around to it?
If you've played as one race more than others, is it because that's your favorite race, or do you like playing as that race better than others, or is it just a coincidence?
For me, I have played as an: Altmer, Breton, Dunmer, Imperial, Orc, and Redguard.
I have not played as, and never plan on playing, an Argonian or Khajiit because I don't want to be a beast. Nothing against those races, they're cool and all, but I can't get into the roleplaying aspect of the game if my character is an animal-person, especially with views so vastly different than humans. I would never know what my character is thinking. Plus I don't want to look at a cat-person/lizard-person for hours on end.
I have not yet played as a Bosmer, mostly because I like Dunmer and Altmer better when wanting to play as an elf, so I just haven't picked a Bosmer yet. I'm sure it'll happen eventually.
I also have not played a Nord, but that's going to change once Skyrim comes out. My main character, the Dragonborn, will be my first Nord.
I don't have a most played race. I've played Imperials, Bretons, and Redguards the same amount. Kind of odd that I've never picked a Nord but have played as the man races so frequently.