Well, scientists wanted to know why, so they started digging. First of all, how did it get started? There are several theories, including one from an expert who says that sensation in the feet and genitals both transmit to the same area of the brain, so there could be some accidental crossover there. But why would it persist through thousands of years of evolution? The answer appears to be disease.
And the tireless efforts of certain visionary directors.
Researchers collected history's records of foot-lust and compared it with the greatest STD outbreaks of the last thousand years: gonorrhea (the 13th century), syphilis (the 16th century, with an encore in the 19th century) and AIDS (the break-dancing years up to now). Scientists learned two things. The first was a greater appreciation for antibiotics, and the second was that foot fetishes are most prevalent in times of disease.
In other words, when our genitals know it's too dangerous to go outside, we start fantasizing about toes and bunions and the way sneakers smell. To test this theory, an intrepid band of sex researchers looked at the prevalence of feet in eight major pornographic publications from 1965 to 1994. Then they made a graph, because graphs are what get scientists off.
You're welcome. Now clean yourselves up.