*shrug* Can’t say I know, myself. I do know that the two regions are right next to each other, given that the roots of the L5 and S2 vertebrae are heavily involved in spreading the thighs (as well as the mechanoreceptors of that area) as well as all actions of the feet. The L4 roots are implicated in many similar actions, as well.
The thing is, we don’t have a concrete answer to what nerves go *exactly* where in the brain, but we do know that it varies between individuals, and that the two regions can, technically, overlap. While nurture may have more of an influence over fetishes in general, erogenous zones seem to be more influenced by nature…though, of course, we don’t know how the people who were tested in adulthood actually appeared as children. Nurture may well have played a role in the way their adult brain became wired to receive signals.
Regardless, there are multiple types of foot fetishists - some being visual (they want to see feet), some being manually tactile (they want to feel feet while seeing them), and some being pedally tactile (they want their own feet touched). Most are a combination of the three, and the reasons for the development of said fetish surely vary wildly from person to person.