a big group of people have always liked feet. It’s one of those kinks accidentally caused by our anatomy - the nerves going out to your feet connect to the spinal cord close to the nerves going out to your genitals, and sometimes the signal travels along both lines.
It’s such a close connection many people, myself included, can feel a particularly strong orgasm in their feet. It’s a very distinct feeling as if electricity running down through your leg, into the foot, and “climaxing” in your ring toe, making it curl and twitch a bit (i also suspect this is the exact reason why the ring toe is associated with love [though idk how farspread this factoid is]). I don’t have a foot fetish but I can definitely see how it’d be very easy to create an association between strong arousal and feet
and per the popularity part - people are just much more open about the kinks they have. To the point where even vanilla people start getting curious about soft kinks like spanking, blindfolds, or fluffy handcuffs