I can't speak for other foot fetishists on this forum, but for me, it began with feet, then spread upward. I think it begins for many with feet because they are more accessible to the libido of a budding tickling fetishist. They're more opportunistic. Their ticklishness often has a more spastic effect on a ticklee than other body parts, possibly because of the soles' nerves extending throughout the body. Thus, the reactions can be more intense and sexy to a tickler. They are probably the most ticklish body part women bare most often while fully clothed, with pits coming in a close second through sleeveless shirts. They easily fit in the hands and are very accessible as massage targets. They are "expressive" unlike other body parts in that they can flex and rotate, which gives a certain stimulating aspect not there with pits, inner thighs, stomach, etc. That might be over-explaining it, but I think all of us fetishists wonder how the hell it all works.
On the flip side, my tickling targets spread naturally through my own love of soft flesh, and legs and torsos have more surface areas for tickling. Furthermore, I suspect many ticklees would agree that they feel most vulnerable when tickled while their arms are restrained above their head, exposing pits, breasts, collarbone and neck to tickling. Feet are very ticklish, but they are not so centrally located, When tickling hands are so close to your main areas for sensing things, I think it heightens vulnerability and intensity. Ticklees also are more likely to be reduced to what I like to call the "quivering mass" after this upper-trunk tickling, which I admit I get off on, as well as any possible embarrassment that comes with that. A lot of the fetish is mental, more than ticklers and ticklees are willing to admit.