Well I think it is because of this:

Whenever we feel ticklish it is really because we are scared about what is going to happen and the body gets curious. If you think the person trying to tickle you is going to pinch you, you get even more ticklish. It's not just the feet, it's all over the body. Even if you know the person will tickle you, you will still be a little curious!

our feet have a larger number of nerve endings in them compared to most other parts of the body, making them extra-sensitive to tickling.

I would guess that the ticklish nature of your foot (or anyone elses) has come around as a precaution against small creatures which may be stood on and cause harm. Say you steped on a scorpion (spider, wasp, etc), then its movements would tickle your foot and cause you to pull you leg away from that place, prehaps this offers enough of a chance of survival to be of use and to be automatic in most people.

Hope this helps