I think foot tickling is heavily tied into foot fetishes, though I'm sure people who don't have foot fetishes can also enjoy foot tickling. I'm guessing a lot of foot tickling lovers are also foot fetishists, and since foot fetish is one of the top (if not THE top) fetishes, that explains the popularity of it.
As a foot fetishist myself, I have to kind of take away my love for feet for a moment and try to look at it as just someone who loves tickling in general. I won't speak for all foot fetishists, but I assume a lot of us like tickling only because we like feet, and that tickling focuses on ONE body part and misses out on all the other wonderful places to tickle. I think it might largely be a visual thing. The feet are far away. All the tickling action is shifted away from the person's core, all the way down the legs. There's a distant, non-personal quality to foot tickling whereas with something like armpit or belly tickling, the face and all the expressions and smiling and laughter are grouped in with all the tickling, and you're RIGHT on top of the person, totally in their personal space, and right in the action.
I can kind of see now why foot tickling wouldn't be as exciting to some if I remove my preference for feet for a second. It does seem kind of boring from a "tickling in general" kind of perspective, but because I have a foot fetish, it's very exciting to me. I will say that I have recently developed a liking for armpit and belly tickling. I wouldn't even say recently developed; I think I've always been a "tickling lover" but foot fetish comes first for me and it took up most of my attention. But now I'm a little sick of it. All it took was one picture of a female UFC fighter posing in a photoshoot in a tank top with her hands on her head, and suddenly, I realized what I had been missing out on all these years and the other desires resurfaced. Now, I want to tickle some pits!