LogFAQs > #960416623

There's 2 things that generally apply to these kinds of fetishes (feet, armpits, etc.) that could explain what causes them to be relatively common.

1. Most of the time those parts were covered and couldn't be seen. People usually wear socks and/or shoes, so seeing exposed feet is rare. People usually have shirts with some form of sleeve, so seeing their armpit directly is rare. That rarity creates curiosity, which leads to attraction.
2. Many times, well groomed body parts were a sign of wealth or comfort. If you had clean, smooth, callous-free feet it meant you didn't spend all day working in the fields or doing hard labor. If you didn't have armpit hair, it meant you could afford luxuries such as razors.
Of course, in modern times these things have become commonplace. Sandals are popular, especially in the summer, so feet are more exposed than they ever used to be in the past. Personal grooming products are also no longer expensive and only obtainable by the wealthy, as well as better shoe design and cheaper manufacturing making it possible to work hard labor without destroying your feet.

So the attraction came from the rarity, and the reason these fetishes seem to be more common now is simply because they're more accessible than in the past. But they still have that residual feeling of "this isn't supposed to be exposed...I'm not supposed to see this."