Maybe it's to do with the crossover of fetishes - Perhaps tickling of feet and sucking of feet are the main thrills for foot fetishists (in addition to being something that appeals to tickle fetishists as part of their enjoyment) - therefore you get a double whammy of it on TMF
Whereas those with belly button fetishes for example don't choose to tickle, but they might lick or poke or fill the bellybutton with liquid and drink from them instead, therefore they don't come on this forum as tickling isn't something they link to their fetish....
Therefore, we have a slight majority of feet talk!
Granted, there are equally ticklish, equally attractive body parts to tickle alongside feet - I find stomach and under arms can be just as erotic and fun and tickleable in a variety of situations, the same way that I find feet are.
I wouldn't want to go near a foot that I found unattractive in terms of its hygiene - but equally, a belly button full of fluff or an underarm that is emitting an unpleasant body odour would also be a no-no.