I think I'm attracted to tickling feet for several reasons.
1) They are the toughest part to tickle. If I want to reach out and poke a young lady in the ribs, that can usually be done with very little effort and I can get a nice ticklish response. But tickling her feet are a challenge. She must be off her feet and unshod. Plus it is a bit dangerous (you might get kicked).
2) There's something amazing about stroking the sole of young lady's foot and causing her to laugh. The foot is the farthest body part from the head and it amazes me that it can have such a profound effect.
3) I firmly believe that TV has something to do with it. In cartoons and old comedies, when someone gets tickled, it is usually the feet that take it. I also find that male characters tend to get tickled in shows more than female (at least, from my own observation), so tickling a young lady is a way for making up for the rarity.
4) Ladies have pretty feet - pure and simple.